How to write a backup script for two alternating USB drives?

I want to use two USB drives with FAT32 filesystems to backup data, but I have a problem with mounting and dismounting the drives and I am not sure how to fix it.

Attached is my first attempt at a backup script.
I think my problem stems from several areas:
1.  Variable declaration $BACKUPDEVICE=/dev/sdc1
2.  Mounting the $BACKUPDEVICE
3.  I think I need to prevent the system from automounting the USB disks to the /media folder so that my script can do it's own mount and umount to the /mnt/usbdrive mount point for the backup job only.

Here is the current mount list:
/dev/md0 on / type reiserfs (rw,acl,user_xattr)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
tmpfs on /dev/shm type tmpfs (rw)
devpts on /dev/pts type devpts (rw,mode=0620,gid=5)
/dev/sda1 on /boot type ext3 (rw,acl,user_xattr)
usbfs on /proc/bus/usb type usbfs (rw)
/dev/sdc1 on /media/usb-160500022Dc5:0:0:0p1 type subfs (rw,noexec,nosuid,nodev,sync,procuid,iocharset=utf8) <=== a double up
/dev/sdd1 on /media/usb-160500022Dc5:0:0:0p1 type subfs (rw,noexec,nosuid,nodev,sync,procuid,iocharset=utf8) <=== a double up
> /dev/sdc1 and /dev/sdd1 are mounted to the same moutpoint!!?

Note from the above that nothing is mounted at the $BACKUPMOUNTPOINT and yet the backups have been running to the $BACKUPMOUNTPOINT and fill the disk!! :-\

What is the best way for me to mount these USB file systems to my $BACKUPMOUNTPOINT with consistency to that it can run in an automatic fashion.  I'd like to be able to simply unplug one disk each morning and replace without all this manual intervention.
Also if it is related, what checks could be written for the second paragraph of my script that may assist help with this problem?

Cheers for anyones help.

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#!/bin/sh
#VARIABLES
 DAYOFWEEK=`date +%a`
 BACKUPDEVICE=/dev/sdc1 #FAT32 USB drive
 BACKUPMOUNTPOINT=/mnt/usbdrive
 BACKUPFOLDER=$BACKUPMOUNTPOINT/backup/$DAYOFWEEK
 BACKUPLOG=/home/$DAYOFWEEK-backup.log
#SUMMARY-BACKUPLOG
 
# Checks
# Is device available for mounting? If not write to $BACKUPLOG
# Is backup disk device already mounted?
 
# Mount the USB disk .
#mount [-fnrsvw] [-o options [,...]] device | dir
 mount -o shortname=winnt $BACKUPDEVICE $BACKUPMOUNTPOINT
 
#Ensure backup folder exists
 mkdir -p $BACKUPFOLDER
 
# Backup data
# /home, /etc ,/var and /root to the FAT32 external USB drive.
# rsync
# -v   verbose output
# -r   recurse into directories
# -l   copy symlinks as symlinks
# -t   preserve times
# --delete delete destination files that don't exist at source
# --modify-window=1 for fat32 compatibility
# no -og options to avoid chown errors
 
 echo "### Backup- /home/data ###"                                        >> $BACKUPLOG
 rsync -vrlt --stats --delete --modify-window=1 /home/data  $BACKUPFOLDER  > $BACKUPLOG
#tail to summary backup log
 echo "### End /home/data ###"                                            >> $BACKUPLOG
 echo ""                                                                  >> $BACKUPLOG
 
cp /home/$DAYOFWEEK-backup.log $BACKUPMOUNTPOINT/
 
# Unmount the backup disk.
 umount $BACKUPDEVICE