I keep getting nagged by notifications about this thread, so... Ok, I'll comment about a few things.
It was my impression that Timeshift was already being criticized prior to my entering the fray. That one comment about it defaulting to backing up the system to the system drive as a bad default is, I believe, my only criticism of the program. I use it. Heck, I've created my own custom icon for the drive which is half way between Timeshift's logo and the Apple Time Machine logo.
And yes, I get it that this being a general comments kind of area, punters as well as pros will see my words and maybe be put off on the program, which in the main is a valuable, useful utility. I'm sorry if that offended anyone. What I would say, however, is that everyone needs to be aware of that particular foible of Timeshift. I also think Timeshift's maintainer(s) need to fix that, and this includes an initial blurb letting the user know they need to connect another drive, along with a further enforcement as previously discussed.
On a separate front, I'm going to re-raise a previously mentioned/discussed idea I've posted that there should also be written for GNU+Linux an equivalent to Apple's Automator program. For those who don't know, it's an incredibly friendly and well-presented front end for creating executable scripts, giving the user access to both building block-style script assembly, and direct basic coding. I'm certain there's plenty of YouTube videos on it for anyone wishing to see what it can do.
Please be polite and remember to mark your fixed problem [SOLVED].
Presently running Linux Mint Cinnamon 19.3.
Know when what you're doing is trying to fit a square peg in a round hole, and STOP.
Still looking for a new job.