|Grub Boot Loader|
Hooked up the hard disk from my old computer to the new one and when it didn't boot I installed a second copy of Linux Mint in a new partition. This evidently fixed whatever was ailing the disk because now it will boot either copy, and when I boot the old copy all the programs I installed and all my files are there still there. Very cool.
I thought about deleting the 2nd copy of Linux, after all it's taking up almost 40 GB of space, but figuring out the steps necessary to accomplish this would mean scanning a bunch of pages of instructions to find the three lines I need and I ain't got time for that. Besides, if it went squirrelly once it might do it again and I'd have to do another installation so we'll just leave it and maybe its ominous presence will be enough to ward off further problems with this disk.
Now all I need to do is convince the boot loader to load the old system instead of defaulting to the new. This requires reading more pages of geeky gibberish but I stumble over something called the grub-customizer, and three commands, complete with all the secret syntax, to be entered at the terminal needed to install it, which I do, and it works, as long as you install it in the right partition which I didn't do the first time so I had to do it a second, but the second time it worked and now it will automatically boot the system I want. So I am happy. For the moment.