Disclamer: This question is
not a Greyhole question, it's a general Amahi/Linux question.
Here's a quick answer anyway:
This happens because your drive is specified in your system's fstab file. (You can google fstab, but put simply it's a file with a list of your system drives/partitions -- either by name or uuid -- and the locations in your file system they will be mounted to.) If one of the drives listed in the fstab file are missing, the system will not start, it's that simple. Why this is the behavior, I don't know. It'd be much better to ignore non-essential missing drives. (i.e. any drive that's not a system or swap volume) but I'm not a Linux developer, and I don't know the reasoning / rationale behind it.
That said, to recover, you need to use a live cd of some kind, mount your system's file system, and edit your fstab to remove the offending drive. (I've had to do this on several occasions.) This link gives a brief description on how to do it:
http://ubuntuforums.org/showthread.php?t=208588 (And FYI, you may be better off commenting out all the storage drives -- and I mean ONLY the storage volumes, it'd be very bad to comment out swap or system partitions -- and reboot until you can isolate which one is actually missing upon getting back into your server.)