Modified uid can no longer see flash drives

[Ralf Mardorf]

You not necessarily need to change the user ID. Add group 500 to user
1000 and group 1000 to user 500, then it usually allows access,
excepted of a few files. It at least doesn't matter if on or the other
group should be already used for some system purpose on one or the
other machine. The user of one machine could get access by group
permissions to the other machine. I don't understand why anybody wants
to give a user the user ID 500 in the first place. You also could
consider to recursively change directory and file group permissions, to
grant access to all files.