fstab is a file defining statical mount points, it wont get corrupted through these exercises
But I don't see your USB device in it. So it might be a permission issue. Devices not in fstab are handled differently, and that may lead to problems sometimes. But before that, lets not overlook the obvious.
Was the device still busy? Something like a filebrowser opened onto your device or a console with the device in the current path, marks the device as busy and will cause the unmount to fail.
And, if as I believe the FreeBSD default is, the write is asynchronous, any illegal or forced unmount will cause loss of data. That is because copying files to an external device (because of asynchonity) is actually not written to device but just set aside as a 'task to do later'. It's not until the device gets unmounted that the write is executed.
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Can't tell an OS by it's GUI
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