[mattack]

I have the mfstools for osx. according to disk utility and devmapper 1.1 my tivo disk is disk3. I type the following:
./mfstool backup -f 9999 -6so /mnt/dos/tivo.bak /dev/disk3
Terminal tells me this:
/mnt/dos/tivo.bak: No such file or directory

because obviously /mnt doesn't exist.

You have to give it an existing pathname.

(I know you're probably not here 5 years later.)