Thanks again, Dan. I ended up starting it before getting your message below, but I think I've got it right. /dev/sdb1 was working and I backed up all of it. /dev/sda1 couldn't be mounted, but they previously were cloned. So I unmounted /dev/sdb, installed GNU ddrescue (on a third drive) and did this: sudo ddrescue -v -n --force /dev/sda /dev/sdb ddrlog.txt After that finishes, I will run this to try to get the bad parts: sudo ddrescue -v -r1 /dev/sda /dev/sdb ddrlog.txt (I'm not sure if I need --force with the second command, but I was forced to use it with the first command.) The first command has been running for 45 minutes so far and it reports zero errors and 100 MB/s average throughput. So far, so good. Mike On Sun, 13 Sep 2015, Dan Armbrust wrote: > On 09/12/2015 03:21 PM, Mike Miller wrote: > Wow, Dan, thanks so much for all the ideas! This is a huge help. Here's what's going on: > > I already formatted the new drive with ext4 and copied 3 TB of data onto it, so I don't want to undo all that right away, but there is another, probably more appealing way to go: > > /dev/sda and /dev/sdb are identical 2 TB drives that were previously in a RAID1. It looks like sdb somehow disconnected from the RAID, but sda kept working for a few months before it > failed. I can mount /dev/sdb1 just fine and I copied all the files off of it onto the new external drive. I had no errors. So maybe /dev/sdb is in pretty good shape. Now that it's backed > up, maybe the best plan is to try to copy /dev/sda to /dev/sdb using one of the dd tools. > > So here's a question: /dev/sdb is formatted for ext4. If I want to use it as the destination drive for the dd copy, do I have to use parted to remove the partition table first? Or what? > > Thanks again! > > Mike > > > You don't have to worry about the partition tables or anything - because rather than copying a partition - such as /dev/sda0 you will just be copying the entire disk - so /dev/sda. > > When you copy the old (failing) disk onto the other disk (that is either the same size, or larger) you will be copying _everything_ - including the partition table, and the formatting info of the file > system. The contents of the disk you are writing to will be completely overwritten. > > So then your replacement disk will be (exactly) what the failing disk was, partition table, labels, filesystem and all. Though, you will likely have some subtle corruption where blocks that couldn't be > read have the wrong bit value. > > So, if you do manage to recover some things - keep in mind that some of the files my have subtle corruption. > > Will you notice if 8 bits are flipped in a 2 GB movie? Probably not... but you would really just have to test the more important files that you recover to make sure they aren't worse than your several > month old backup. > > Dan > > >