I think that you can safely add your drive to fstab, I think there's an option for partitions that are not always present, possibly 'nofail', though that has been known to cause problems, like read-only file-system, so proceed cautiously
The mountall(8) program that mounts filesystem during boot also
recognises additional options that the ordinary mount(8) tool does not.
These are: ``bootwait'' which can be applied to remote filesystems
mounted outside of /usr or /var, without which mountall(8) would not
hold up the boot for these; ``nobootwait'' which can be applied to non-
remote filesystems to explicitly instruct mountall(8) not to hold up
the boot for them; ``optional'' which causes the entry to be ignored if
the filesystem type is not known at boot time; and ``showthrough''
which permits a mountpoint to be mounted before its parent mountpoint
(this latter should be used carefully, as it can cause boot hangs).
``optional'' which causes the entry to be ignored if
the filesystem type is not known at boot time
which I suspect could be combined with a filesystem type of
auto and the
bootwait option , in adition to
nofail to cause the system to load the drive if present, and to fail and proceed if the drive is unavailable.
consider: man mountall and man fstab