The internal memory is overwritten when you install a new ROM.
To clear off an SD or CF card, open a terminal...
rm -rf /mnt/card/*
rm -rf /mnt/cf/*
Those two commands will completely wipe your cf and sd cards of all data. Is that what you want?
If so, be careful with rm -rf ... warning running it in the wrong spot will be upsetting if you don\'t have a backup!
