Linux – Exchange data between two identical hard drives

I've done this with known good USB enclosures and operated the drive while it was inside the closed freezer (with my laptop on the kitchen counter).

It rarely helps, there are so many forms of failure, that it's a crapshoot. The Data Recovery experts tell you not to do this, but "seek professional help". How that help got to be professional is by reading and trying stuff with disks they didn't care about.

The most awesome recovery I managed to date was to open a drive and unstick the spindle with a screwdriver and pliers. Shockingly, it ran for a few hours while I recovered most of the data. I didn't even use a cleanroom. I did that after the freezer trick failed.

The most hopeless recovery I tried was a Mac disk where a head delaminated. There, running the disk or trying the freezer trick just continued to destroy the medium. A DR expert would have been able to do a partial recovery by sourcing identical drive mechanics and mechanically transfering the platters. My amateur efforts just destroyed the disk.

Not having the right tools can be a problem too. After my disk froze a second time, I tried to unstick it with the pliers and screwdriver again..... I slipped. The platter was hoplessly destroyed. The DR guys have proper tools to do this and minimize risk.

Best of all the DR guys have the experience to identify the problem without running the disk and listening to it click while your medium is scratched to hell.

If the data is worth anything, it needs to go to the DR guys. If not, the freezer is a good third-to-last effort.

After the freezer I would try the drop-test.

Then try a home-made cleanroom from plastic bags, tape and a clear box. Pop open the drive and see if the heads are gone, if htere are big scratches or if the spindle just won't turn. Be sure to have all your clean tools and your clean drive inside the box before you start. Also a cable to be able to connect the disk to your computer.

Most of this is just learning about disks at this point. Possibility of recovery is remote.

That's a bad idea because you're deliberately degrading your RAID and Resyncs might fail unexpectedly. It's better to hook the new disk up to the system (so you have n+1 disks) and then use mdadm --replace to sync it in. That way the RAID never degrades in between.

You don't have to fail / remove drives to find out which is which. You can see a device's role number in mdadm --examine, in mdstat output [UUU] in role numbers is [012]; and you can check the drive's serial number with hdparm or smartctl and compare to the sticker on the drive itself.

For partitions, it might be better to use GPT nowadays instead of MSDOS. If you are not only replacing disks but also upgrading them in size, you might have no other choice anyhow, since MSDOS partitions pretty much stop at 2TB.

Personally I don't do this at all. So what if the disks are 3 years old? Disks live a lot longer than that, and new disks die all the same.

It's much more important to test your disks on a regular (automated) basis, and replace disks once they have their first pending/uncorrectable/reallocated sector, read error in selftest, or other issues.

Even more important is having backups of any data you don't want to lose.

You could also switch to RAID6 for more redundancy, but the case of two disks dying at the same time is highly unlikely as long as you actively check for errors. Don't let your rebuild be your first read test in years.