I’m curious as to how this has turned out. Just in case it’s still pending, here’s my $0.02.
I agree that you should transfer the files over to your own drive, then delete them from the ‘clients’ drive.
If they want the files, I’d be tempted to add a ‘carring charge’ on top of what they already owe. Settle for half? No way. Payment, how about a bank wire transfer directly into your account. To heck with checks.
If these guys are really going belly up, they should know that they aren’t about to get the work you’ve done without a guaranteed form of payment.
Hope it will work in your favor if it hasn’t already.