If the gui prints a error message like

Examination failed (Cannot unmount disk)

try the following commands:

1. first stop services

   /etc/init.d/services.sh stop
   /etc/init.d/opentftp.sh stop
   /etc/init.d/Qthttpd.sh stop
   /etc/init.d/rsnap.sh stop

2. detect the mountpoint - use df or mount, e.g. /dev/mapper/cachedev1 or 2
3. try to unmount

   umount /dev/mapper/cachedev1

4. check if unmounted

   mount | grep -i cache

5. check filesystem

   e2fsck_64 -p -f -v -C 0 /dev/mapper/cachedev1