I've got a mid-2013 11" MacBook Air that I fixed up.
It had no SSD when I got it but I've been running it off an external USB 3 hard drive fine for about a year and a half including backpacking around the world for a year.
But in the last few weeks the whole OS has been prone to crashing. Sometimes after a couple of days uptime, sometimes after only a short time.
I'm trying to narrow it down to a bad motherboard or bad hard drive.
Normally I wouldn't expect a bad hard drive to crash the whole system but just report various errors and warnings from the OS. Or if it were to cause a crash I would expect the OS to detect it most of the time and display some kind of dialog on the screen before restarting.
Often though it seems to crash when I'm moving the computer or the external drive or touch the cable. Most of the time moving it does not cause a crash though.
I have tried both USB ports so I can rule out damaged ports. It could be something to do with the cable or the drive though.
But if anyone can tell me for 100% sure that a bad hard drive or cable is ever known to crash the whole OS I can try to find another drive to test.
Likewise if anyone can tell me for 100% that a bad hard drive can never crash the whole OS I can focus my attention elsewhere.
Once or twice after a crash it restarted with the flashing folder icon but started up fine next time. I never got any errors about the hard drive from the OS or any app. Checking the drive with the OS tools finds no errors. The drive has both Mac and Windows partitions and I've tested on both OSes. I haven't yet seen any problem using the drive on my Windows laptop but I mostly use the Mac now.
The only causal factor I can think of is that one day I put the Mac in my backpack with the external drive still attached. The Mac will sleep but not hibernate due to the OS design. I was jogging for an hour or two and when I opened my backpack the drive cable was no longer plugged in. Plugging or unplugging a USB device wakes a sleeping Mac and waking the Mac will cause the hard drive to start. Bumping or shaking a running hard drive can damage it. None of this should've had any effect on the rest of the Mac hardware I can think of.