There is always the option to leave the mail on the server for POP3 systems, and IMAP is server-based anyways (right??), but that would likely hit a mailbox size limitation. I understand that keeping the mail on the server isn't feasible at all times, but that is truly the best way to sync mail content across multiple locations. Lacking that, make sure that Outlook is closed before copying the PST to your flash drive to move it about and things should be OK.