If you have attached a drive with a damaged volume since you started Data Rescue, Data Rescue may not automatically notice it. Try the Expert - Refresh Volume List menu item. To access the Expert menu on the toolbar, you'll need to enable the Expert Features in Preferences.
If the volume you are looking for still does not appear, it may be because Data Rescue is unable to find the correct name for it, in which case it may show some other name such as unknown for the volume name. Or it may be that Data Rescue can see the device, but is not able to recognize a volume structure on the device. If that is the case, you will need to choose the device name for scanning rather than the volume.
Finally, it could be possible that your drive is malfunctioning to the point where your computer is not able to talk to it at all, in which case even the device name will not appear. In this case, no software will be able to scan your device. In this latter case, you might try some or all of the following things to see if your device can be made to appear: Double-check the drive cables and power source; remove/reattach the drive and/or power cycle it; power down and restart your computer.
You can double-check that the drive can be seen by your computer by using Disk Utility to see if it can detect the drive. If the drive fails to show up in Disk Utility, this means that the computer does not detect the presence of the drive, and no software will be able to access it either.
If possible, you could also try mounting the drive into an external enclosure and then see if the computer will detect the drive, we've seen this work in some cases. We've also seen some hard drives appear intermittently depending on the nature of the problem with the drive, so you may want to give it several tries, shutting the computer down in between, to see if the drive will re-appear to the system.

