with external drive connected and mounted, it won't boot if that somehow disappears
Couple ideas, for what little they may be worth. Grub might be installed on the wrong drive. Or, if you’re getting past grub, look up systemd’s nofail option for /etc/fstab.
I remember what it was now, I had mounted in fstab, but using the device path (e.g. /dev/sdb1) instead of the UUID of the disk. The device path seems to occasionally change (which is bizarre as it is the only device connected). Using a UUID does the trick. But it was just an example of many minor details that needed to be fiddled with in order to create a 24/7 laptop server. I don't mind - it's part of the fun I guess, but I'm just waiting for the next edge case.
Couple ideas, for what little they may be worth. Grub might be installed on the wrong drive. Or, if you’re getting past grub, look up systemd’s nofail option for /etc/fstab.