Need suggestions on a backup issue

[Paul E Condon <pecondon@mesanetworks.net>]

I was bitten by a bug in my backup system recently and would like
advice on how to fix the problem. I use Bash scripts and crontab to do
a daily backup each morning at 6:25 AM. (Why then? Because it is the
default time set up by Debian on a new install, and I've never seen a
reason to change it.) The script runs on a computer that is separate
from my main desktop computer and also serves a print and apt cache
server. The script writes the backup directly on to a USB drive that I
keep plugged into the computer, and uses rsync to only write backup of
files that are new or have changed. Occasionally, I swap out the USB
drive with an alternative one and keep the swapped out one in a safe
place. I had thought the system was working fine until a few days ago
when this backup computer ran out of disk space on its root partition
and crashed. (The root partition is 20GB and the system files use 3GB
of it.) The cause of the crash was some how the USB drive had become
umounted and the daily backup script filled the root partition with an
attempt to backup all 100GB of stuff on the main desktop computer.

Debian already provides an email facility that allows cron scripts to
email me with error messages. I need a way to test in Bash whether, or
not, there actually is a disk mounted at /media/wdp8. 

What test can I code in Bash to determine that there is no disk
mounted at this mount point? I'm thinking of testing for the
presence of the file lost+found, but is there something better?

Thanks
--
Paul E Condon 
pecondon@mesanetworks.net