Are you trying to mount
/etc/fstab available only on the other disk? Can you see the flaw now?
I guess you need
/etc on the root disk, so (separate!)
fstab can be read and eventually mount the desired
/etc from the other disk. Other files in the initial
/etc may be in use before the desired one is mounted, so you would need to maintain (a stub)
/etc on your root filesystem anyway. This could be a mess.
systemd you can mount without
fstab, yet the configuration still lives within
There may be workarounds. This question on Unix & Linux SE is somewhat useful: Moving
/etc to separate partition.
Anyway I would keep things simple and not mount
/etc separately. Normally it stores so little amount of data it should fit in the root filesystem. If there is not enough room in
/etc then you shouldn't use Ubuntu (which is not a tiny distro) in the first place.
/etc separately only adds problems, solves nothing. Drop the idea.