Actually, a drive can experience a "head crash" that can result in a loss of some data. The heads that read the data are "flying" above the surface of the disks at a height of only a few thousandths of an inch. If the disk is subject to a hard jolt while the disk is spinning, then heads can crash onto the surface of the disk. The area where the heads contact the disk are damaged and the data stored there can be lost. If the damaged area contains part of the OS, the system may be unable to boot.
However, it would take a lot of head crashes to lose a significant amount of data from the drive. And as long as the drive will still spin up, you can place the drive into another system, do a bit-by-bit copy of the drive onto a new drive, and then recover the data from the undamaged areas of the drive.
Even if a head crash were to have happened on Lois Lerner's hard drive the emails would have been unaffected.
In a corporate or enterprise level infrastructure, data like that is not stored locally but on an Exchange (or other) server and also on a storage array with drives mounted for that server and also mirrored onto other arrays and then exported to tape or some other media which is moved offsite for archive.
The IRS thinks that no one knows this. If Congress were to start threatening IT people, they would start coming out of the woodwork...probably with the wanted data in hand.