[Bug-ddrescue] Selective recovery

[David Morrison]

I have been given a WD Thunderbolt Duo device which has intermittent 
problems. From what I can see, it connects two SATA disks (3TB each) 
to a Mac using Thunderbolt.

However, OS X allows multiple disks connected to a Mac to be set up 
using software RAID. The owner was told this one had been set up to 
mirror the two disks, which theoretically gave it a capacity of 3TB 
with a certain amount of redundancy.

Unfortunately, it turns out to be set up as a single 6TB volume 
striped across both disks.

And even more unfortunately, I have discovered that one of the disks 
has a lot of bad blocks. The 6TB volume mounts fine, and a lot of 
files can be copied off it. However, a lot of files give errors in 
the Mac Finder. If possible, I would like to recover those files 
(family photos) so was thinking of using ddrescue to try to recover 
those bad blocks.

Now I can see two approaches. One would be to copy the failing 3TB 
disk, then try to put the RAID volume together again using the copy 
and the other original disk. Is the RAID system likely to accept this?

The other is to copy the entire 6TB volume, depending on whether 
ddrescue can work on a RAID volume. Can it? (And I would have to get 
a 6TB disk.)

I would be grateful for any thoughts on the best approach, and 
anything else I need to know.

Thanks