repoman
Contributor
how do you do that with a mac?
If you can get to Linux/Unix command line utilities, you may want to google DD DD allows raw writes bit by bit. So if you had a hard disk to save, attach a second USB hard disk and us DD to copy what is readable from disk A to disk B. If disk A is failing, you may not be able to recover all of hard disk A. There are Linux live CDs such as Clonezilla that allow such things in the PC world, I don't know what is considered good for such rescues in the Mac world. DD's bit by bit copying does not rely on any specific file system, so it will also copy corrupted files, but also won't halt if for some reason it doesn't understand a corrupted partition table et al. So it may be a mess to sort out even if it saves data.
If the hard disk is too far gone, this won't work. If the disks are sound but the hd's electronics are bad, this won't work.
If this was me, I'd not try to access it again until I had considered my options and was ready to implement a plan.