Why would a drive spontaneously from being /dev/sdb to /dev/sda? mode

[Chris Green]

I recently was doing some [re-]configuration of my Lenovo Thinkpad
running [x]ubuntu 21.10 and it failed to reboot.  After a bit of a
worrying interval and lots of failed boots I found that an added disk
drive that has been /dev/sdb1 for a long time (a year and a half or
so) has become /dev/sda1.

Since /dev/sdba (or now, /dev/sda1) isn't actually necessary for the
system to run I just commented it out of /etc/fstab and the system now
boots OK.

However, some questions:-

    Why did it decide to change from /dev/sda to /dev/sdb?  Yes, I
    know I can avoid the problem by using a UUID for the drive in
    fstab but it would have been nice if the problem hadn't happened.

    Why didn't the system boot?  When I tried to boot in maintenance
    mode I could see the 90 seconds timeout ending but nothing
    happened afterwards.  Surely, after the timeout, the boot
    should/could continue?  ... and again, yes I know there's options
    to add in fstab to say ignore mount failures.


-- 
Chris Green