I couldn't resist - so dug out my packed XT.
I exported a track, route and associated waypoints to a gdb file. I did this from both Basecamp and from Mapsource.
There was no way that I could get the XT to locate any of the data that was in the GDB file either when I placed it in internal storage or on the memory card - which is what I expected.
Look at the screen shots of parts of the same file - one exported as a GPX - legible text, the other exported as a GDB - largely unreadble and containing what appear to be database data structure information - eg field names, which were never used in the data that I created.

- gpx.jpg (41.82 KiB) Viewed 3451 times

- GDB.jpg (64.54 KiB) Viewed 3451 times
GDB is the format that was used in Mapource. But the same format is also used by Basecamp - and it seems not to have changed. Export data as a GDB file, and Mapsource will read it. Ditto the other way around.
When Basecamp came along, it introduced the folder and list structure, and when BAsecamp is opened it autoamtically opens up the last GDB file that was opened. Changes are saved to the database as you make them - there is no need to save before closing down.
But Basecamp can open different databases. I have over 30 of them from the last 15 years or so. Typically one for each year and another for experimenting or testing. You can create your own in the Edit / Options / General. If you want to know how to do this - See P25 of the Zumo 595 and Basecamp documents. Downloadable from here - first post.
viewtopic.php?t=521
Coincidence. I was just about to download it to check the page reference and noted that the Zumo 590 and Basecamp pdf document has been download 590 times. Seemed a shame to download it again to spoil that coincidence. This is a screen shot - not a link !!

- Zumo 590 Downloads.jpg (11.87 KiB) Viewed 3451 times
Have owned Zumo 550, 660 == Now have Zumo XT2, XT, 595, 590, Headache
Use Basecamp (mainly), MyRouteApp (sometimes), Competent with Tread for XT2, Can use Explore for XT - but it offers nothing that I want !
Links:
Zumo 590/5 & BC . . .
Zumo XT & BC . . .
Navigating with Zumo Booklet