So, to put this all in one post, rather than having you read all of the tweets: I got a new external hard drive the other day, and plugged it into my Windows 10 desktop. I copied almost everything over (with the exception of a few folders I don't think need to be backed up) from my old drive to my new drive. I then switched their drive letters (so the new drive is now set to the same drive letter the old drive used to use, and the old drive has a new drive letter). Then, I set up SyncBack to keep my new drive backed up on my old drive regularly. Having both of those drives plugged in at the same time, presumably because there's some sort of system file stored somewhere on them that's getting duplicated or that Windows is finding in the wrong place, caused it to completely geek out and basically stop working. Unplugged the old drive, and things *seem* to be back to normal again.