Hello, just ran into this.
I had the same problem and found no real resolution except to run both
the following two commands (one after the other) at the command line, and that mounted the stupid device so I could move/delete/copy/etc the files from Nemo (and the command line, for that matter). Since I have to do this once a week or so on more than 1 device to update the files, I put these in a simple Bash script, like this:
sudo parted -l
sudo lsblk -f
It's a stupid fix but it's been working on my system for a few years now and I got tired of wasting time looking for more reasonable solutions on this device.
By the way, in case you or someone else also runs into another SanDisk problem, these devices also have problems with audio books not always playing in the correct order.
This problem didn't show up until a family member began downloading audio books from sites like https://librivox.org
Others appear to have found other fixes that didn't work for me (I don't remember what those were, but I found them in forum discussions on this problem).
What did work for me was finding that the errant SanDisk device only plays in timestamp order (not alphabetically by file name or mp3 title, for example). Some of these audio books have many tens or sometimes a hundred or more files - a real pain to update by hand.
So I wrote another Bash script to change the file time stamps in the correct order. In my case, I just iterate through the files, using the touch command to modify the time stamp, then the sleep command to pause for 61 seconds, then touch the next file, etc. Like this:
for x in * ; do
echo " .. Updating modified timestamp for $x, and then waiting $SLEEP_SECONDS seconds to do the next one.."
Can I equate "stupid" and "SanDisk" together again in this post? I use a smartphone and other devices for podcasts, etc., but sometimes a little device like the SanDisk was a better fit for something I or someone else in the family was doing (e.g., I use mine for listening to audio books in bed to fall asleep. MIL uses hers on the train to listen to old tv show audio when traveling). It's nice to have something that doesn't break the bank if stolen, lost, or damaged, and I've purchased more than a dozen of these kinds of devices over many years for family use.
However, struggling with the stupid time stamp problem broke me. I've gotten so disgusted with how little SanDisk has cared to fix such stupid problems even after people have been complaining about them for years on SanDisk's own forums that I'm no longer replacing these devices for myself or family members when they do finally die.