But you gave a good tip Markus. I tried alot like "mount -t vfat -o uid=500,gid=500 /dev/sda /mnt/sda" (or -o rw) or right clicking in the gui and changing rights there but got a "block device is not writable" I'm using Knoppix 3.7 and mtab gives
/dev/sda /mnt/sda vfat ro,nosuid,nodev,umask=000,uid=1000,gid=1000 0 0 after booting.

Edit: Write access is temporary, after each transaction ( ex deleting a file, deleting 100 files) it fails the next one.
Umounting and remounting makes it work again for 1 transaction.