What they do depends on what they find. Generally, they open up the drives first, without having applied any power (to prevent doing any more damage).

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I'll disagree with Tom's methodology a bit. If the recoverer looks at the controller board and spots a failure point on that, s/he won't open the drive immediately. Opening the drive requires a clean environment which costs money. Logically, s/he would identify the failed component. and work out whether it was on the basic I/O side or the ATA/IDE side. If it is on the ATA/IDE side, swap the controller board. If it is on the I/O side, swap the platters -- expensive.