install backup menu , let it load ,connect your n900 to your pc, press "w" , do whatever repairs or format the first partition on the emmc .
if it gets mounted automatically on your box , unmount it by issuing
umount /dev/sdb1 , replace sdb1 with the correct name in your situation .
don't forget to format it as fat32 . I recommend that you use gparted for any modifications to your partition .