All has been pretty much fine until the last couple days.
First I got a warning about low disk space. I encountered this before and was unable to determine what to delete, since almost everything I create or download in Mint gets saved to the external drive and therefore shouldn't be taking up any space that Linux Mint needs. When it happened before I got rid of some big files that were in "Trash," which thankfully seemed to fix the problem, because I was at a loss as to what else I could remove! I've used a few versions of Mint but in some ways I'm still a relative newbie, with limited understanding of the Linux file system.
This time, unlike last, there wasn't much in "Trash" to delete. So I watched helplessly as another warning said I had even less space. Things were also getting extremely slow. I was running Firefox, where I have hundreds of open browser tabs. (I know; I can't seem to control myself. But they're organized by the wonderful Tab Groups add-on.) I closed Firefox and the warnings stopped, but everything was still extremely slow.
Then I rebooted, hoping that would fix the slowness. Instead it took longer than usual and then, after seeing the Linux Mint logo and hoping to see the desktop, I instead saw a black screen for at least half a minute, followed by the worst warning yet: This computer has only 0 bytes disk space... and the suggestion, again, to delete or offload files. Although I still didn't know where I was going to look for stuff to delete, I tried to select the "Examine" button but then realized I didn't have a mouse pointer. I couldn't even select "Examine" with my cursor keys and press Enter -- there was no response. I had to force a shut-down using the power button.
On reboot I selected a different flash drive this time, and opened Ubuntu. I explored the drive containing Linux Mint looking for something to delete, but again saw nothing obvious. I found a Trash icon, but because I'm pretty inexperienced navigating the Ubuntu "Unity" desktop I was confused and unsure where those trash files were located. Still I deleted a 144 MB folder within, but on returning to try Linux Mint again, got the same "0 byte" message at bootup. I later determined that trash had resided on the computer's internal drive, which explains why there was no benefit to my Linux Mint flash drive. Also, on the warning message this time there weren't even "Examine" and "Ignore" buttons. Again I killed the computer with the power button (which I'm loathe to do) and returned to Ubuntu.
I assume it's the drive that contains Linux Mint 18.2 where I have to delete or remove something -- is that right? And if so, what? I can't access anything from within Linux Mint since that's toast, but using Ubuntu or another distro I can see files inside a YUMI-created folder called linuxmint-18.2-mate-64bit, including (along with a few tiny files)...
- casper-rw, which (if my understanding is correct) contains all my settings and everything I've downloaded which I haven't put on an external drive. But there's no way to delete individual data from casper-rw, correct?
- the Linux Mint iso file which YUMI used to create the Linux Mint portion of the flash drive. That iso file is 1660 MB, but I don't know whether it can be deleted now, nor whether that would help.
- Somewhere there must also be a big Mozilla folder, due to all those Firefox tabs. I've found it in the root folder of Linux Mint before, and periodically backed it up, but I always had to show hidden files first. I don't recall whether I tried to find it using Ubuntu and right now, in Puppy Linux, showing hidden files reveals only a small Mozilla folder (about 40 MB) so that must be for Firefox in Puppy, not in Linux Mint, which was typically closer to 300 MB. I don't know if that's counted toward used disk space, but in any case I can't find it now, perhaps because it's on the same flash drive as Puppy, the distro I'm IN, preventing me from exploring that drive? Or maybe it's part of the casper-rw file.
Since my post is rather long, I highlighted the questions I'm seeking answers to, so nobody will have to wade through all that a second time. I sure would appreciate some help. I feel lost without my familiar features and the projects I was working on in my browser tabs.