Here's a copy of a post I made about 5 years ago on some forum when I did the same thing. I have a Montana 600 but I doubt that anything has changed.
I assume you can get the profiles and voices, etc. from Gamin Express. Of course, you have a backup of your waypoints, etc.
Good Luck
It'a alive! Garmin support was spectacularly useless and I refused to pay Garmin $160 to format a 4GB drive so I fixed it myself. So, if you accidentally format the internal storage of your Montana here's how to get it back.
1. Download and install RMPrepUSB. I found version 2.1.725
2. Connect Montana via USB and use force mass storage if necessary to get a drive letter assigned.
3. Launch RMPrepUSB and select the Montana internal storage in the list at the top.
4. Enter MAX as partition size.
5. Enter GARMIN for Volume label (all caps)
6. Check “Set partition as non-bootable”
7. Uncheck “No user prompts”
8. Select radio button “MS_DOS bootable....”
9. Select radio button “FAT32”
10. Chck “Boot as FDD (A: no MBR)
11. Click button “Prepare Drive”
You should now have a usable internal storage for the Montana. Disconnect the Montana and let it boot normally. The Montana firmware should now see the storage and create a few directories and files on the storage. You now need to locate your backup and copy over the Profiles, Voice, Vehicles, etc.
Good luck and spend your $160 on something fun.