How to recover data from malfunctioning or dead HD?

[roccenstein]

I replaced the HD in my mom's PC a while back because her pc wouldn't boot and it wouldn't allow me to do a clean install or repair install.  After I replaced it, she let my uncle take the old HD and he later told her there was nothing wrong with it and was able to pull all of her files off.  Is there some trick to this?  No matter what I did, I could not get her computer to start.  Even after starting it with a boot diagnostic disc, I wasn't able to locate any of her files, so how was he able to?  Any ideas?  I'm just looking to learn in case I'm faced with this again in the future...

[Geek-9pm]

It just happens.
Consider the following statement:
Failure of electronic devices follows statistically profitable trends.
That is true, but does not tell you way a drive was dead and then came back to life. The statistics apply to a large set of failures. Individual failures are hard to asses.

If you need as rational theory, here is one. The drive motor 'stalled' and could not turn o get to  speed. When you moved it, some slight mechanical vibration freed the motor and it began to turn easily.

That may not be what  really happened.  Your desire to accept this story is called 'rationalization'. 

Really, do you want more speculation? Does it really help?
Anyway, you can not make a good rule from just one incident.

And yes, this does happen sometimes. Be glad you got the date back.

[Salmon Trout]

When you moved it, some slight mechanical vibration freed the motor and it began to turn easily.

Yes, or maybe pooled lubricant was redistributed following transport of disk.

[Uselesscamper]

Perhaps your Uncle used a Boot CD with tools that assisted in the recovery. A Linux USB or CD boot drive will often be able to recover data you can no longer access via Windows.