It's possible, but I'm not sure if it's worth that much trouble.

The entirety of the GPO5 library is under 12gig.  GPO4 is less than 4gig.  So unless you are really short on drive space, consider just installing the whole thing on your system drive. 

There is also a free demo pack of some of the Garritan Library instruments that you can find at the Plogue Site, which is quite small and might contain enough base instruments to meet your needs in situations where you don't want to use the portable drive (you could install the demo pack on your System drive for a test run...if it's not enough, just uninstall it).

First, do you have Finale on the system?  I ask because later versions of that come with a smaller complete library that might meet your needs when you don't want to carry a portable drive around with you.  Finale also has a nice Expanded GM soundset that can work without ARIA at all (but it only works with Finale).

If you're determined to weed out just a few specific instruments from any Garritan Library...you can see exactly which samples are needed for a given instrument, and where they are kept on your hard-drive by locating the sfz file and looking under the <region> tags.  For the most part each instrument has its samples kept in the same directory.

Each instrument in an ARIA based Garritan Library has an sfz file that goes with it.  The sfz file contains instructions called opcodes that tell ARIA how to load and play samples.  These files can be viewed and edited with a text editor, and you can even experiment and change them a bit to create some custom variations on your sounds (be sure to make backups of the original sfz files, or a mistake will mean you must reinstall the entire library).  Some of the newer GPO5 instruments support #include lines in the sfz file, which allows ARIA to parse several different text files into one...so be aware that in some cases you might need to be sure that several different sfz files are kept intact, and in the same directory structure on your target 'lean' installation.

Be aware that you can't edit the bank.xml file(s) for a Garritan Library without possibly messing up the registration.  I'm pretty sure ARIA does some sort of md5 file check to make sure you've not altered that part of the library each time you launch it.  With that in mind, realize that even instruments that you've removed will still SHOW UP in your ARIA lists of available instruments.  So, you'll need to take care NOT to try loading any of the instruments you've removed.  If you do try to load an instrument that you've removed his sfz file or samples, then you're going to get error messages.

To make it work in a way where you can fairly easily swap between your full installation on the secondary drive, and your lean installation on the system drive, I'd suggest setting up a filesystem junction (or symbolic link for Macs) on your system drive that points to your portable drive.  Reinstall the whole Library to your junctioned locations on the System Drive.  This will make software always think you're running Garritan Libraries from Drive C.

Note, if you're not sure what a file system junction is on Windows, or a File System Symbolic Link is on a Mac, then this might not be the way to go until you research the topic a bit online.  Essentially, these are methods to create a kind of shortcut on the hard drive that tricks the OS into getting a file from a different device and/or directory.  They are a little different from a 'short-cut' however, in that it's done at a much lower level in the OS, and software just thinks things are running from the device containing the junction pointer.

Next, you'd make another directory on your system drive to COPY whatever  'portions' of the library you intend to use into.  Keep the same directory structure as found on your external drive.

With this sort of setup, when you want to change over to your stripped down version, you'd change the file junction to point to the directory on your System drive (remove it, and re-create it pointing to the new location).  When you want to go back to the full library on your external drive, you'd again remove the junction, and create a new one pointing to the library on your external drive.

Having said all that...if you're that short on drive space, I recommend just getting a nice, new, LARGE capacity drive.  Clone your System Drive to the new one, then swap them out.  Now you've got plenty of space for your 'on the go' sample libraries.  If it's a laptop, and battery life is important, then you might want to shell out the extra bucks to get a nice SSD drive (as large as you can afford).  Samsung and Intel are two very reliable brands known to host streaming samples very well.  If SSD is out of budget, then I do recommend a faster 72000 spindle drive with good seek times (Maybe a WD Black, or equivalent).

These days, you might even find USB sticks large and fast enough to hold a Library or Two in its entirety.  While it's still an extra bit of gear to carry around...it's a lot less than a bulky external drive with a cable to hook it up.

Thanks for the detailed answer. How do I find out what version my gpo is?

If you are on Windows, you can examine all of your installed programs, and the installer versions will be listed.

I.E.  Tap the windows key.

Type "progr" and click on the "Programs and Features" icon in your start bar.

That should bring up a list of pretty much everything installed on your PC.  You can then scroll to find your installed Garritan Libraries, or even search/filter the list.  The installer version should fill you in.

Other differences:

Anything GPO4 and newer will use the ARIA player.  Older libraries might use different players (such as Kontakt or GigaStudio).

If you know where it is installed on your hard drive have a look.  Unless you forced some other names when you installed the library, the top levels should have names like:  "Personal Orchestra 4" or "Personal Orchestra 5".

If you check the size of the top level of the directory:

GPO5 will be a bit under 12 Gig.

GPO4 will be less than 4 Gig.

GPO5 has a bunch of extra choir instruments such as Boys, Children etc, where as GPO4 only has one or two choirs.

GPO5 has an extra bank of String Instruments listed as "Garritan Orchestral Strings".

GPO5 has different Pianos listed, "Such as Steinway Intimate and Resonant D and B models".  While GPO4 will just have "Steinway, Duo, and Lite" versions".

GPO5 gets more organs and harps as well...most obvious extras are the "Custom Console" organ, and the new 'pedaled harps' (Each have special UI Macros in the controls tab where you can build up your own custom organ ranks, and harp pedal modes).

You could browse into the library on your hard drive and examine log files.  Typically the installer leaves some clues as to what library it has installed in a *.log file.

Yes, it is possible to have both GPO4 and GPO5 installed on the same system at the same time. 

If you're not trying to use older GPO4 soundsets with a host (such as the old GPO4 soundset for Sibelius) then you can uninstall GPO4, as the library is pretty redundant to GPO5.

Thanks again, I think I have GPO2 lol. Do you think gpo5 is comparable to vienna symphony library or east west?

I don't have any EW or VP plugins, but the price difference is rather substantial if you acquire the full range of instruments included in GPO5.

In my opinion, based on years of usage, For $150 USD, it's hard to find a better Orchestral library.

The price for GPO is more than fair, and the quality is decent.  It doesn't ask for much in the way of system resources (even with larger scores).  It doesn't take much disk space.  It doesn't need a usb key/dongle.  You can hear demos and such as to what is possible in an open tracking DAW in quests for more realistic mock-ups.  Note, it's not likely to sound this good 'out of the box' in something like Finale or Sibelius...you'll have to shape the tracks with Continuous Controller events.

The only other Symphonic Library I have tried that is more or less in this price bracket is Steinberg's Halion Symphonic Orchestra.  HSO doesn't cover as much ground (number and variations of instruments, I.E. NO HARPS) for the money.  Some instruments in HSO I like better, and some much less than GPO.  It depends on the score's demands really.

Without question you can find larger and more advanced orchestral libraries, but they're going to cost more, need a more powerful rig to run, and will still take some 'practice' to learn how to get 'realistic' sounding arrangements.

Some years ago I got the big Garritan Ultimate Collection, as I also needed the Wind/Marching and Jazz band stuff.  I'm still getting my money's worth out of the purchase several times over.  In my case, it was money well invested.

Thanks man. How do I approach learning what I need to know to make it sound realistic? I am just beginning this vst stuff, and there are so many settings (effects, reverbs, velocities, and stuff). Which settings do you think give the most bang-for-buck?

Good question, and there are probably many approaches. 

What kind of DAW will you be using?  Do you work with a score package at all (Finale/Sibelius/Dorico/etc.)?  If you generally compose in a scoring package, then you'll have a pretty good starting point already.  If not, there will be some extra steps for roughing in the instrument setup that'll seem rather time consuming at first (but you'll learn lots of time saving tricks as you go).

To begin with, I'd recommend grabbing some scores of pieces that you are pretty familiar with.  Start with simple scores (I.E.  Just a string quintet, or Classical Symphonies such as Beethoven are good in that they're pretty simple in terms of instrumentation).  Listen to some of your favorite real recordings of them on the SAME SPEAKERS and system that your DAW will use, and pick a recording who's master sounds nice to your ears that you'd like to acclimate your ears towards attempting to duplicate.  That gives you a 'reference point' for whatever speakers or headphones you've got attached to your system.

Now, load that score or MIDI file into your DAW and start experimenting :)

With GPO, it's mostly about setting up the attack and subtle playing style changes to sustained pitches and dynamics that you want for a given style. I personally do this part LAST with a pretty dry mix...which is going to sound very harsh and tinny until you give it a simulated stage and room presence.  Note, that before you add some reverb, it's going to sound horrible to most people's ears (especially brass).  In fact, this is what often confuses people about Garritan libraries...the samples were made close miced, in a soundproof environment.  This 'dry quality' is good in that YOU get to later lay it into whatever type of room and mix you require (instead of having a given room married to your samples with recorded reverb tails, which can't be changed).

Some of the GPO instruments offer key-switches to allow bouncing among different articulation styles or playing techniques.  I.E.  From arco to pizzicato, or martele, or spiccato, and so on.  You can also load individual variants of these articulations and simply control them all with separate tracks (or channel bounce).  Even with your arco sounds, it won't be uncommon to keep several versions loaded on a track so you can divert notes to it from some other track when you want a subtle change in timber/style for a given passage.

The main CC you'll need to work with is the mod wheel (CC1)...you'll typically want changes to this for pretty much every single note on the score.  Many Garritan Orchestral instruments also respond to key_velocity to get variations in the sound of the attack.  Next comes understanding and application of the legato switches.  Wind/Brass/Strings have a legato switch that allows you to change the attack of a note so it is more appropriate for slurred or connected passages.  Next comes various changes you can make to timber or dynamic settings that a given instrument might have (You can see them all, as well as what CC can control them in the control panel of ARIA).  Finally comes pitchbend or portamento settings.  With GPO, most of this is done by entering CC events on controller lanes in your DAW.

At first, focus on small sections of a piece, and do the work manually (draw in each CC you think you'll want) on controller lanes in your DAW.  You can count on needing a lane for your key_velocity, CC1 (dynamics), and either CC64 or CC68 (your legato switch). 

Don't forget that groove and tempo has MUCH to do with making a piece more realistic.  A quantized piece with a steady tempo is going to sound rather machine like.  So be liberal in changing the tempo, and doing the sorts of groove variations that real orchestras have (I.E. do they rush or drag certain beats, or hit a particular beat with some implied or unwritten accent, etc.).  Humanize your note attacks and releases so they don't always fall on the same MIDI tick...this alone will have a pretty big impact on making your mock-up sound more realistic.

Eventually, you'll get to know all the instruments pretty well in your sample library, and you'll have a pretty good idea of where things should be to rough in a piece, and you'll be able to use scripts or logic editors in your DAW to do alot of mass edits to an entire score in just a few clicks. 

I.E.  In Cubase, I can use a special logic editor to throw a CC1 event on the time stamp with every note-on, and match it's value to the MIDI note number.  That makes volume higher as a melody goes up in pitch, and softer as the melody trends down.  From there I can run sections of my piece through logical editors as needed to balance things, and even randomize the attacks a bit.  Next, I'll emphasize certain beats if needed using my logic editors (example:  increase the velocity  and CC1 of every 2nd and 4th beats of bars 17-32 a certain percentage).  Finally, I'll go in and enter hair-pin and sudden dynamic changes (like sfz) and give individual notes extra attention if they need it.

Most DAWs will have some sort of scripting or logical editor that you'll want to master if you don't want to have to tweak EVERY NOTE one by one with your mouse and keyboard.  If you're looking at buying a first DAW, and orchestral mock-up is a goal, make sure it has scripting or MIDI logic editing in the toolbox before buying.  Most of my tracking DAW experience is in CuBase, and it has really good MIDI editors, as well as simple booleen logic editors that you can call up and build on the fly (and also save for future use, or even bind to keys or MIDI events as macros).  Some others, like Reaper may have a better scripting engine under the hood, but this might have a bit of learning curve to learn to use at first, and may not have as many tools in the UI itself for batch or scripted MIDI editing.

Once you've got dynamics, attacks, and subtle pitch changes entered in ways that at least give an impression of how a 'real player' or section of players for that instrument would do things, you'll build a seating arrangement (pan and volume) and shape up the virtual room (reverb).  Out of the box, GPO has a pretty decent seating arrangement assigned by default, but you'll most likely want to make changes depending on the size of your mock-up orchestra, and the virtual stage and room you're putting them in.

Building your virtual orchestra comes into play as well.  You'll be able to try different variants of instruments, and layer up to get the size and type of sound you're going after.  You'll get a variety of 'tutti' or section instruments, and also solo player and small group instruments, as well as 'overlay' instruments.  This allows you to build your orchestra piece by piece/player by player if required.

GPO comes with a basic set of reverbs built into ARIA that you can use if you don't have your own reverb plugins.  Each instance of ARIA gets two reverb units in parallel, where one is a standard reverb (for building stage presence) and the other of the convolution type (for room signatures).  Each instrument can optionally have some shelving EQ (the mid-range frequency node is adjustable).  These are the tools you'll use to blend and warm-up your mix.

Once you feel like you've done all you can with the built in tools, you can use other things your DAW to go even further.  I.E.  You might have reverbs of your own that you like better.  You can apply various compressors or other dynamic shaping plugins.  Apply stereo imaging, and the list goes on.

T-T. I might need to hire you one day. The learning curve seems steep