Please see thread with subject "Be careful when editing /etc/fstab" for a bit of background.
My computer had two swap partitions, on two different disks, when
one of them started to generate CRC errors, seek errors, etc.
Once I determined which of the two it was, I commented out the /etc/fstab entry for it, halted the system, removed the faulty disk and rebooted. I got dumped into the "emergency rescue" single user mode, after waiting several minutes for various "start jobs" to time out.
Given the above referenced email, and the info in the links it references, I would have thought that rebooting, with the modified fstab file, should have worked just fine.
However, as noted, it did not, so I must assume systemd has a unit (is this the right term?) defining my original swap setup.
I did find in man pages, that if both fstab and a systemd unit define the same thing, systemd wins.
That just reinforces the idea that somewhere, systemd knows the swap disk devices.
But I cannot figure out where this might be, or even if this is the correct interpretation.
Any suggestions on next steps greatly appreciated.