Disk Drill lets you backup any media (disk, memory card, etc) into a disk image (DMG). This is quite convenient for later recovery, especially if the disk you need to recover from is failing.
In cases where the disk image you created cannot be mounted (viewed) on your Mac, or you are getting the “no mountable file systems” warning, you can try to attach the non-mountable disk image with the help of Disk Drill.
Go to Disk Drill’s File menu and select the Attach Non-mountable Images option. Then navigate to the location of the disk image file, select it and click Attach. That’s it. Now you can continue with your recovery procedure.