Upgrading PS4 Pro to SSD, update file cannot be found

Solution 1:

So it dawned on me, that I kept being told that the "USB Storage Device Is Unavailable". Usually "Storage Device" refers to an external HD, connected to the rear port on the console. For the hell of it I put the USB key in the rear port... and immediately it worked FINE where it would not work in either of the front ports!

To be clear both front ports work fine, they charge controllers, accept USB keys for media, etc. But for whatever reason, in Safe Mode, they won't work for a USB stick (or the rear is somehow overriding). So if this ever happens to you, dear stranger on the interwebz... try the rear port!