Click the "Check Database Integrity" button in the Advanced pane of the Zotero prefs. If it reports an error, run your database through the DB Repair Tool. You're using Firefox 3, which fixes database instability problems in Firefox 2, so theoretically you shouldn't experience any further corruption.

If that fixes the problem, I'd then recommend making regular backups and checking the database integrity every so often. If it continues to report problems, please let us know. Use of a network drive such as iDisk hasn't been extensively tested, though many people have reported using such setups without trouble.

If "Check Database Integrity" doesn't initially report any errors, the problem may lie with iDisk. The actual error message you're getting is "database is locked".

Using any sort of automated external sync process with the Zotero database is rather dangerous, since the external process has no way of knowing when the database is in a consistent state (which it won't be if Zotero is writing to the database), and any changes made to the database externally while Zotero is open are quite likely to corrupt the database.

This is much of the reason we're working on internal server sync support. However, Zotero 1.5 will also by default obtain an exclusive lock on the database while it is open, which should help prevent problems like this with external tools. (We may periodically write out an unlocked backup file so that tools like Time Machine can still back up a copy of the database even if Firefox is left open.)