I've got the problem on two devices, and it seems that the FAT is corrupted on the user partition and the device in this case took forever to start an update.
When I say forever, it take for one more than one night, I plug it at around 19h, and found it back at nearly 10h updated and restarted into USB connection.
So do a fsck/scandisk/chkdsk on your device before trying to update it.
Be sure to always cleanly eject your device before removing the USB plug!
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