Day 2 of "Linux does not like this one flash drive in particular."
Just the fact I can say it is one flash drive particularly, and that is not Nemo (Linux Mint's file manager), is all progress.
I routinely considered the events of note preceding the great fuckening: I had accessed my flash drive on Windows 10; I had unmounted the flash drive but pulled it out before it had completed unmounting; and I had done a hard shut-down when my boot-up slowed to a crawl (thanks to NordVPN, which initially I blamed for this entire clusterfuck).
I accessed the flash drive in Windows and ran a repair function on it; it claimed the flash drive was fixed (Nemo said, "I'll be the judge of that"). I also changed permissions on the flash drive to everybody and their dog. Windows was fine with all of this, as well as all reading and writing of every kind. Windows is very cool with my flash drive. Very chummy.
In Linux, I changed permissions on the flash drive itself; then in Nemo. I discovered that my file manager is named "Nemo" and that there are other ones I can test. I tried "Nautilus" and "Thunar." This is when I realized the problem is probably with my flash drive: other people with my problem reported that Nemo was the problem child, and that the other file managers would work where Nemo didn't. Neither one would write to the flashdrive.
I noted that if I left Nemo open while waiting for a document to transfer, a second dialogue box would pop up, frozen in the act of transferring the file. If I shut down my OS I would sometimes get an error message from Files telling me that it was busy.
Something is corrupted as balls.