Data recovery with GNU ddrescue

ddrescue will create a image of a disk while skipping broken blocks on the disk, when finished, mount the image with hdiutil, then run DiskWarrior to repair damage to the filesystem in the process.

sudo screen ./ddrescue /dev/rdisk4 /Volumes/Internal\ IT/zeller\ temp/ddrescue.dmg ~/mapfile
hdiutil attach -imagekey diskimage-class=CRawDiskImage /Volumes/Internal\ IT/zeller\ temp/ddrescue.dmg