I guess I just took it out, i.e. didn't use the "Safely Remove Hardware " option
I think that might have been the problem. It seems that sometimes you can 'pull' a solid state device without consequence; but apparently there are circumstances when doing this can cause the card /usb stick to be unreadable device thereafter.
I've had this happen to me once, and nearby friends managed to corrupt a couple of their SD cards containing 'once in a lifetime trip' pictures by apparently not being aware of the need to formally go through the 'safely remove hardware' steps. They got most of them back by engaging a very expensive data recovery service. Since this, I've been very astute about following this process to the letter.
I suggest that even if it can be salvaged, it is advisable to put the affected card in the bin, and get a new one to replace it. They are very inexpensive - especially when compared with what a roll of Kodachrome might have cost these days ...
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