In the Adagio 81A releases, we added functionality to the backup function to accomodate the WebSafe application. In making changes to the backup function, we see that backing up to a removable drive indeed incorrectly gives the error message you have reported.
This has been passed along to development in order to make a fix. We apologize for the inconvenience this has caused.
For now and until we release a fix, you'll have to backup to a different location and then move the file to your ZIP disk.
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Regards,
Softrak Tech Support