How do data recovery firms recover data from a "dead" hard disk?

Solution 1:

Here's a general high level view:

1) If platters are spinning and a BIOS can identify the drive try software. They will likely a 'low level' software program to see if the data is recoverable. Sometimes they get lucky and Steve Gibson's SpinRite will make things work. :)

2) If platters are not spinning and/or the drive is not recognized by BIOS then they will likely replace the controller board on the drive. If they have one in stock this is pretty easy. They will then test if that changed anything.

3) The desperation move is to remove the platters. This is not trivial and is only done after other options fail. The platters will be put in another drive and the data will be attempted to be removed.

Much of what you pay for with drive restoration is the expertise, their huge stock of drives and controllers, and the clean room. Those make running a drive restoration service fairly capital intensive.

Solution 2:

It depends on what's wrong.

In some cases, the part that's "bad" can be replaced (in a clean room), or the platters can be moved from one drive to another (again, in a clean room).

Worst case scenario, I have heard of microscopic evaluation of the platter surface, though I can't imagine what that would cost.