Finale 27.4, Windows 10, Sonar, (Cubase), Dorico Pro & Dorico iPad
How much RAM and what sort of processor does you iMac have?
Did you ever mention the format of the graphic file you exported? TIFF? JPG? PNG? It might make a difference.
Paul, please post a screen shot so we can see the problem. (ALT+PrtScn to put the top window on the Clipboard. Paint to paste, edit, and export your screenshot.)
Also, even if you have GPO, GIfF sounds include a selection of sounds from other Garritan libraries (JABB, GWI, etc.) which you may find handy.
GPO4 has more sounds (and more options within instrument groups) than the included Finale Garritan sounds (GIfF sounds). https://makemusic.zendesk.com/hc/en-us/articles/217663758-Setting-up-Garrita...
And some folks find better results downloading if they use a browser other than Chrome.
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These days, the "media kit" is a Thumb Drive instead of a DVD.
It sounds as if you are in Page Layout mode when opening your file. Those margin and system margin lines appear whenever you are in Page Layout mode. If that is not what you are seeing, check the...
Locking systems is not the default. Either your import added it or you did (possibly as part of something else you did that implied locking).