The GPO installer certainly does contain the audio data, in AudioSamples/personal_orchestra_5/Harps/Lyons_4006. (These files need to be uncompressed in the installation process.)

The files get installed to {your GPO samples}/Harps/Lyons_4006. 

If the files are missing, ARIA should flag an error message when you select the instrument and it tries to load them. If there is no error message, then the files are loading, and there's some other reason why they're not playing.

I've reinstalled a couple times - the .audio files are nowhere on any of my disks and drives; this is true of a couple other GPO5 instruments, too. There are literally no files to manually navigate to or relocate. To be sure, this is probably a result of both the MacOS move to 64 bit (which bricked my boot SSD) and Finale's (which bricked an SSD data drive - which I know to keep persistently and constantly backed up to magnetic media, none of my scores and arrangements were lost). Much reinstalling from scratch ensued but not everything in Finale/Aria/GPO5 has worked as expected since - and it has made me fear any further upgrades from Finale or Apple. 

In the case of the harp, my solution is to stack on of the basic KS files (those, from the same GPO5 installation, work) with a VPO harp (sfz) on the same channel which yields a fairly convincing result. But I have no idea of how much GPO5 Grand Concert Harp would be better. : (

And yes, I get error messages on the affected files. (I couldn't edit my response - not sure why.)

That's very weird, because the installer does have those files and works here. Do you have a Time Machine backup that might go far enough back to when they were present?

How have you searched for the files? I wouldn't rely on Spotlight to search much outside the user home. Is it possible that the installer is putting the files somewhere, but ARIA is looking in another location, which remains incomplete?

ARIA looks for the samples in the locations contained in this file:
/Library/Preferences/com.Garritan.Personal Orchestra 5.plist

I think you need to be careful about ascribing cause to an effect: ARIA has been 64-bit for several years, so turning off 32-bit support in Catalina won't have affected GPO at all. I'd put money on Finale not being the cause of a disk hardware failure; and I'd be very surprised if the OS did, either!

And software can't brick an SSD.

It may not be possible, but I have two bricked CD's nonetheless.  - one instantly after the Apple installer finished and relaunched. The other immediately after updating Finale and Aria, then launching Finale - but I think that is an artifact of whatever it is the MacOS update did. (One cannot rule out the SSD manufacturer.)

Disk failure often occur after OS installation, because it involves hundreds of thousands of writes and erases, and many Gb of data, so it is often 'the straw that breaks the camel's back'. But a healthy drive should easily withstand the process. 

Boot drive was new - it had to be to make the Mac switch from 32 to 64 without reformatting the original drive, the data drive was somewhat used. I'm going to chalk it up to Heisenberg's dog.

Your post says you're running Mojave: in which case you can still run 32-bit and 64-bit code. So I don't know what you mean by "making the Mac switch to 64-bit" (unless you mean installing Catalina). But just updating to Catalina does not require a reformat of the disk, so I don't know what it is you did. 

But back to ARIA: take a look at the plist I mentioned to see where ARIA is expecting the sample files to be. Then compare that to where you told the last installer to put the files, and check what is in each.

I have other income-earning software that went 64 bit, the principal reason for the update. In terms of music (and the Mac) I can access more RAM on 64 bit as well - makes a difference in Cubase where I mix my finals, and I also have more RAM available in Aria racks as well.

If the plist file you mentioned is supposed to reference the location of GPO samples, all that is in that particular file is a license path to "Personal Orchestra 5_license.png."

Here's mine:

.
You'll see the 'sample_dir' keys give the locations of the samples for each bank.  If you don't have that value, then that would explain why you're not getting any samples loaded.

Macs have been running 64-bit apps since 2009.  The only change is that now they ONLY run 64-bit. Before Catalina, they ran 32-bit app and 64-bit apps.  Apple made the whole bit-ness largely invisible to the user. 

Nothing like that here. Here's the entire file.

I see that is the User library. The system library notes this path - and indeed, that is where I have been looking for the .audio files that do not exist on my system. Note: I have never gotten the Grand Harp to work - the mystery is a handful of other .audio files that vanished.

OK. If there are no audio files at:

/Users/Shared/Garritan/GPO5_Samples/Harps/Lyons_4006

then the most likely explanation is that the installer is saving them to some other location. First, check whether there isn't a GPO_Samples folder inside GPO_Samples, because sometimes when reinstalling, you can select 'GPO_Samples' as the folder where the installer should install GPO_Samples!

After that, I would recommend using something like EasyFind to search for "Lyons" across ALL your disks and see whether it can find the folder. It will sit and actually search through everywhere, so may take some time. (It's free, and very useful generally.)

https://www.devontechnologies.com/apps/freeware

That EasyFind app is pretty good - though in the end it comes down to something interesting: there are two kinds of folders, 'Samples' and 'Samples Data.' There are no .audio files associated with Lyons_4006, but EasyFind did locate a whole bunch of pedal mappings to .sfz files in Application Support. (Two images.)

Yes, those are the instrument  definitions, which define which sample should play for what note, which key switch triggers which effect, etc. 
SamplesData is the installer folder that contains compressed versions of the samples, which are unpacked in the installation process. 

If you have downloaded the latest GPO5 installer (which was revised for Finale 27) and run it, and you're still not getting all the files installed, then you need to flag a support ticket with MM. If there was a problem with the installer itself, then there would be lots of other similar complaints.

FWIW, I don't have a folder called "x-deprecated" in my installation.  And you've got SFZ files where the samples should be, but they should be  in /Library/Application Support/Garritan...., as set out in the plist.

I would suggest deleting everything:
Users/Shared/GPO_Samples,
/Library/Application Support/Garritan
all the Aria and Garritan plists in /Library/Preferences and ~/Library/Preferences
Then reinstall, and you might get a better understanding of what's happening without any previous clutter.

I may try that - but it's what I've done before. This may proved to be an excuse to buy the $64k Mac and start all over.

: )

I can report that Grand Harp is much better than whatever I was using before.