i had similar problems before.
my problem was caused by this:
- an SD card formatted using the PC can only be mounted via /dev/mmcblk1 as fat/vfat.
- if I use linux fdisk on the SD card to create partitions, & format /dev/mmcblk1p1, then i have to mount /dev/mmcblk1p1. mounting /dev/mmcblk1 would fail.
(all above actually makes sense if you know a bit of how disk contents are written).

so, i updated some system scripts in my n900 to handle all those cases properly, including BackupMenu.
also updated scripts to recognize ext2 partitions.
attached original scripts & updated scripts (to be placed into /usr/sbin).
no warranty provided.
use at your own risk!

warning: if you don't take precautions, you run risk of bricking your n900 permanently because some partitions can't get mounted. first be sure you know how to unbrick your n900, then try.

opps. the attached BackupMenu replacement needs to be in its proper place. & do check that the original matches before replacing!