A Google search indicates that mtmfs is Mobile Time Machine file system daemon.
I think that might be for the snapshots Time Machine makes for laptops when they aren't connected to their Time Machine drive. Try turning Time Machine off in System Preferences when you aren't connected to your Time Machine drive.
Disable TM then use TimeMachineSceduler (http://www.klieme.com/TimeMachineScheduler.html) to set it to do a once a day backup when you are connected. I've been using this for about a year since I installed my second drive in the CD bay without problems. TM will only backup changes btw, full backups are only performed the first time.