I have hooked up an external USB3 Samsung T5 SSD to my iMac model iMac14,2, formatted it as APFS, and installed macOS High Sierra on it, to use as my primary system drive.
I can use the system for a week, maybe a week and a half, at a time, but after a while my machine will hard freeze as I am using it - as in, can't move the mouse or type with the keyboard, screen is frozen. It usually happens when I'm doing something disk-intensive, like opening the fuzzy file browser in Visual Studio Code, or opening a file browser, but I have a suspicion that it has frozen and then rebooted on its own while I was away from the system at least once.
I checked the console and didn't see anything jump out at me in that span of time where the system froze. I ran First Aid on the disk itself and the disk container, but the only warning I got was warning: Overallocation Detected on Main device: (7723705+1) bitmap address (10e2e)
The only other thing that might be relevant is that something that I noticed that happened once or twice in the past week is that my keyboard suddenly stopped working - no keys would cause an action - but my mouse that is hooked up to my keyboard would continue to work. Strange.
I am currently using three of the four USB ports available in the back - one has the Samsung SSD, another has the keyboard/mouse hooked up, and a third has my Time Machine spinning-rust HDD hooked up to it, which does not have its own power cable.
Option, Command, and Esc (Escape)and then FORCE quit apps one at the time