I have had a bunch of times when you need to use a hard drive designed for a Laptop in a desktop system, say for formatting, or copying massive amounts of files. Laptop drives are by spec. 2.5inch wide, while desktop drives are 3.5 inch wide. This isn't a problem for the average user because we have these nice 3.5 to 2.5inch adapters, however what if you wanted to put a full scale drive into a laptop? You're screwed. Of course, in most cases you couldn't even fit the full drive into the laptop just due to it's actual size, not just the different adapter.
But why do this, it's going to be so ugly! Easy... because I have a pile of 3.5 inch drives, and all my 2.5 inch drives are dead. I personally don't care how it looks, assuming it works.
The pinout between a 40 pin ide (standard) and 44 pin ide (for laptops) are identical except for the last 2 columns on the connector:
pin 41 +5v (Logic)
pin 42 +5V (Motor)
pin 43 Ground (Return)
pin 44 TYPE- (0=ATA)
These actually won't be used to power a standard sized drive due to the increased voltage needed over a 2.5" drive, plus 3.5" drives typically require +12v as well. We will just be ignoring these extra pins, and taking power from somewhere else.
Step 1: Making the PCB
As I recall, the first 20 columns of pins are exactly like the full sized cable, the last two are for the +5v dc for the "fun-sized" drive. Unfortunately, it doesn't have the +12v dc which fill size drives need, only +5v dc. I made solder points for them, but the weren't actually used.