I assumed you were somewhat familiar with linux commands. Here's an explanation:

su # become root

mkdir /media/hdb6 # create mount point "hdb6" inside directory "media"; assuming mount point doesn't exist already

mount -o rw /dev/hdb6 /media/hdb6 # mount hdb6 and make it writable; clicking on the desktop icon will mount the drive "read only"

mcedit /media/hdb6/etc/fstab # launch editor

umount /media/hdb6 # unmount partition when you're done