Etc/fstab won't retain edits
From MEPIS Documentation Wiki
In MEPIS, /etc/fstab has two portions: a static one and a dynamic one. If you want your changes to be retained make sure you edit the static portions (the first part of the file).
You can also move lines from dynamic to static portion in order to make a change permanent (for example move the entry for your USB drive from dynamic entries to static entries, or if you want some drives to be mounted automatically change noauto to auto and move the entry to static portion of the file - See: Automount drives)
To edit the file you need to be root.