I unmount my drive and it still happens.are you sure?
try again, and this time right click on the drive icon and unmount it before disconnecting it.
also, if archiving data for long periods of time (well over a year roughly) I'd advise using a conventional (ferromagnetic) hard drive. Solid state needs to refresh itself every so often to prevent bit rot, and this requires it be powered on every so often.
Did you double check the complete file path, down to the drive letter, when saving?I unmount my drive and it still happens.