Cron problem!

[Hongbo Ma <hma@wisc.edu>]

I wrote a script called full_backup.sh to do bakcup. It is supposed to
run at a given time everyday. I log in as root, and type "crontab -e"
add a entry like following to the crontab file:
*******************************
15  18   *    *    *    /usr/local/bin/full_backup.sh 
********************************


The content of the full_backup.sh (I put it under /usr/local/bin) is:

******************************************
#!/bin/sh
mt -f /dev/nst0 rewind

rm /var/log/tar/snapshot

tar -cvpf /dev/nst0 --label="Full Backup Set Created
on `date '+%d-%B-%y'`." --listed-incremental=/var/log
/tar/snapshot /home/hma/project
*******************************************

I can run this script with no problem manually and get my archive file
on tape. However, if I use cron, although it IS  triggered by cron, the
'tar' command in the script didn't run successfully. That is to say, I
know the script was executed by checking the consequence of
"rm /var/log/tar/snapshot", but I didn't get the archive file on tape.

Strangely, if I changed the backup target directory from  the
"/home/hma/project" to "home/hma/proposal", then every thing is fine. I
get the archive file on tape. "proposal" directory is about 3MB, while
"project" directory is about 700MB. But the size should not be a
problem, since I can manually run the script to backup "project"
correclty.

Obiousely, something is wrong with my cron, but I just don't know how to
correct it. Currently, I don't have sendmail program installed on my
system, so I can't get the email cron is supposed to send to me. 

Any help is highly appreciated!!

Hongbo Ma