Flash drive suddenly Read Only, unable to completely clear Firefox cache, “Error extracting files”

[TechTerrorized]

It all started with downloading a large, 709.5 MB, Zip archive of FLAC audio files, (from the Internet Archive). With our slow connection, and the horribly underpowered old Compaq NetBook I was using, it took an hour and a half. (Compaq Mini CQ 10-100, originally Windows XP, now running Linux Mint Debian Edition4 32 bit.) I was downloading to a flash drive, and extracted the folder with files to the same flash drive. It said, “Error extracting files”, something it often says, and I haven’t been able to find an explanation of what that really means and what to do about it. I’ve simply been checking that the files are all there, and if not, fetching the missing ones singly. I don’t know why the downloaded files have larger MB numbers than are given on the dropdown lists for FLAC and VBR MP3 files under “Download Options”. I figure perhaps the error message is triggered by the MB discrepancy.

In this case, the two files, two sides of an LP, were there, with the usual larger MB count, but when I tried to Rename them, and when I tried “Properties”, I was told that I didn’t have the permissions to do that, and “Error setting permissions: Read-only file system”. Same thing when I tried to Delete the Zip archive, the folder with its two files, or the files separately. I tried Rename and Delete also on our better computer with the same result.

So the 64 GB flash drive with 11.8 GB space is now Read-only, and I can’t find what could have caused it, and how to, or if it’s possible to, undo that. (Micro Center 3.1 64 GB, five for $27. on Amazon, where the top reviewer neatly explains how they are, in effect, “factory seconds”.)

I hope it’s even safe to use it, to play all the music I’d downloaded to it, and copy files to disc. Would it be safe to play those two sides of that last record, or should it be avoided? I wonder, if I were to copy everything, except that last Zip archive and the folder and files extracted from it, off the now read-only flash drive, if it would even let me erase-reformat it, and if it did, if I should use it, or is it defective?

Another consequence, it would seem, of that download, is that there are 5.3 MB in the Firefox browser cache that will not clear. I found instructions in Mozilla support for how to delete the Firefox cache folder, but the file manager can’t find any such folder. I tried uninstalling and installing Firefox, but it kept my settings, and also that 5.3 MB of something in the cache. Why would something in the cache not clear? Can it be cleared?

So, to reiterate the questions:
1. Why would a flash drive – USB stick – suddenly become read-only?
2. Would that be an indication that the flash drive is defective?
3. Can it be restored to read-write?
4. Could it have been caused by a corrupt file, presumably corrupted in the long download?
5. Would the corrupt file also be what is refusing to be cleared from the Firefox browser cache?
6. Can that cache ever again be cleared to “0 bytes”?
7. Will the flash drive allow itself to be reformatted?
8. Is it safe to use the flash drive, either as is - read-only, for the files on it, or if it were to be reformatted? And would it be safe to play the two files of that last record, sitting there, undeletable?
9. What do you do when you get the message, “Error extracting files”? Do you extract again, and do you have it overwrite whatever files were extracted the first time, or “Skip” those? Do you just check that the files are all there and seem to be alright, and figure the error message was a false positive, perhaps caused by Checksum discrepancy?

Sorry for submitting such a long set of different but related questions, which include probably wrong notions misinterpreted from answers to similar searched-up questions, and are not necessarily LMDE-centric. I will be grateful for any answers, as will subsequent searchers, since previous discussions to be found online are not very helpful.