Mountain Lion Time Machine can't reliably connect to LaCie 2big NAS
I have a LaCie drive running my Time Machine disk as well (Firewire connection). when I upgraded to Mountain Lion, I had similar issues as you are explaining.

Go to System Preferences, and click on Energy Saver (lightbulb icon under "Hardware"). On all Macs running Mountain Lion, especially the one connected to the LaCie NAS drive, make sure to uncheck "Put hard disks to sleep when possible".

This should fix your issue.