Two repairs: both failed internal drives. One MacBook and one Macbook Pro.
Painful. but I should first say that everything that iFixit sent and all of the repair videos and advice were perfect. The actual physical replacement of the drive (in both computers ...) was a complete snap.
The black MacBook was never repaired because it seems the drive was never the problem, but there is no way I could have known this without the tools and drive enclosure that I got from iFixit. Once I hooked up the "failed drive" externally using the drive enclosure, it worked. The new drive would not work when installed, so obviously the failure is upstream, and I don't have the time to figure it out (this computer was just going to be a "bonus" as it was long given up for dead--I thought to resurrect it only because I was already ordering stuff to fix my "important" computer: the macbook pro which ....
Was a complete pain. The issue was that my drive was failing, not just being upgraded, so the SuperDuper plan was not going to work. I cloned my old drive with Data Rescue 3. But the clone, once put on the new drive (and yes, I formatted and partitioned correctly) would not boot. ProSoft (makers of Data Rescue) point out that the do not intend or guarantee the drive to be bootable--cloning is meant just to save all the data you can. The next four days were filled with frustrating iterations, attempts to "bless" the clone, reinstalling OS's, etc ...
Finally, I hit on backing up to Time Machine while placing my old badly failing drive in the disk enclosure (it would boot from this position, just not internally, or I would have done this first). I then restored the new drive from Time Machine and only lost a little data to corruption ...
If your disk will not boot, it may be the drive, but it may also be something else. A data rescue program (if you do not have a very recent backup) is a good first step (unless the data is crucial, in which you case you should immediately hand over the drive to professionals--BEFORE defragmenting or doing anything else.). Then, once you have created a clone, and you can be sure that the vast majority of your stuff is safe, see if you can create a time machine backup by using an OS install disk to select the disk and "backup to Time Machine".
Also, a drive enclosure is a very useful diagnostic tool. If your drive has disappeared or won't boot, try removing it and running it as an external drive. Does it boot? If not, can you access (and save) your data while using it as an auxiliary drive? Or, as was the case with one of my laptops, is it totally fine? If so, there is a hardware problem upstream from the drive.