I have a 2008 iMac in which the drive is failing. SMART status seems ok although AHT reported 4hdd/11/40000000 SATA (0,0)
which is the code for disk failure AFAIK. Normal boot fails, Firewire target disk mode works, however after a certain amount of disk access the drive stops responding – for example in the middle of a large file transfer it never get beyond a certain point. After this folder listing stops working and all I can do is reboot the iMac.
I have read similar questions that suggest using Ddrescue. However the behaviour of my drive is that it doesn't report failure, it just never finishes. Will Ddrescue cope with this?
Update: I've made the question title less specific so hopefully more people find this thread – be sure to check all comments below for some useful tips.
Best Answer
Since you're having problems with the drive after using it for a while, no tool may be of help completely. Depending on what's actually failing on the drive, trying to re-read the same sections for a long time could possibly aggravate and expand the problem to other areas. So I would not recommend ddrescue as the first option.
Since there may not be a single solution that is guaranteed to work for this case, I would suggest a combination of different approaches:
Try to copy small sections at a time (starting with the most valuable data, leaving aside OS, Applications and Application Preferences that can be redone later with some time on hand) and give it some rest in between.
Try the freezer trick to see if it helps
For any data that you are able to copy out, do a quick verification and back it up elsewhere. You can never have too many backups!
Depending on the drive's manufacturer, download the hard drive test tool from the manufacturer's site, install it on a USB thumb drive and run it. This may help in detecting the actual issue better while at the same time marking bad sectors so that they're hidden (the latter is possible without these tools as well).
As a final measure, use ddrescue to see what you can recover before throwing or recycling the drive.
In the future, ensure you regularly use
Time Machine
in conjunction with a disk cloning utility (Clonezilla, Carbon Copy Cloner or SuperDuper!)