Can I attach a .dmg without mounting the drive that it is sitting in?

I have a .dmg sitting on an external drive (which also contains other data). The .dmg is an image of a failed drive so I can't mount it but I want to attach the .dmg _without_ mounting the partition it is in. Is this possible? I ask because, if I mount the partition, attach the .dmg and then try to run TestDisk, TestDisk returns this message:

Write access for this media is not available.
TestDisk won't be able to modify it.

- No partition from this disk must be mounted:
Open the Disk Utility (In Finder -> Application -> Utility folder)
and press Unmount button for each volume from this disk

I tried what I thought would work:

attach /dev/disk2s2/cfp/disk0s2-clone.dmg

it didn't,

failed - Not a Directory

I guess a different way of asking my question is, "How can I use TestDisk on a .dmg if I can't mount the partition that the .dmg is in?"