You cannot transfer drive ownership between Workspace instances, not just in GAM, in the UI too.
There really are two options:
1) Do it as a copy instead of a move
2) Use shared drives, as follows:
a) Create a shared drive on the destination Workspace instance, b) share it with the user being moved on the source Workspace instance
c) On the source Workspace move the files belonging to the source user into the shared drive
d) On the destination Workspace, move the files back out of the shared drive.
And yes, both are painful. Extremely. Sorry to be the bearer of bad new!
Ian
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Ian Crew
Architect, Communication and Collaboration Services
Productivity & Collaboration Services
Berkeley IT
University of California, Berkeley