Hi John, Have you had a look with diskpart at the drives from command line?
I'd already thought of that one but, like everything else on a Windows machine, diskpart does not see the faulty memory sticks (although it does see any working stick, or SD card, that I plug into the machine.
I've had this happen on both mircosd and usb thumb drives that have been tampered with to present higher capacity than actually available.
What usually happens is the low end of storage becomes entirely un usable and doesn't report anything. Doesn't automount, doesn't show in file manager nothing............ but within diskpart it appears as hardware with a completely screwed front end.
I should perhaps be a little clearer about what happened, since, with both of the sticks, everything initially worked fine. I copied a hundred or so files, totally some 200-250MB onto the stick from my laptop, and then could happily access and open those files from the laptop (even after unplugging and re-plugging the stick). I then plugged the stick into the tablet, and could see all the files on the stick. I then successfully copied a small batch of the files from stick into the laptop *(onto its microSD card), and the tablet could then see, and open that small batch of copied files, now on its microSD.
It was when I tried to copy further files into the tablet that everything went wrong. To be honest, I can't remember details of exactly 'what happened' but, suffice it to say that I could not copy more files. I therefore removed the stick from the tablet, to 'examine' it on the laptop and, ever since then, neither laptop nor tablet have been able to see the memory stick.
Exactly the same happened with the second stick. I successfully transferred all the files onto it (from laptop) and successfully copied one small batch of files from it to the tablet (where they could be seen and opened), but then, just as with the first one, it 'died' (well, 'became invisible' - and also invisible to other PCs and laptops (all Win 10 other than one PC running XP).
With the third, similar but older, memory stick, everything just worked 'as expected', and I ended up with all of the files successfully copied to the tablet.
Kind Regards, John