Here is how I went about it, which might help you if you want to make one yourself...
When you pick a drive to use, make sure it has four conductors going to the motor ("Y configuration") rather than three ("Delta configuration") if you want to use the TDA5140A/TDA5144 to drive it. The conductors are likely to be on a tiny flexible ribbon going into the motor on the back of the board.
Get yourself a proper Torx driver to take out the screws holding the drive together (you can work around it with other things but you will drive yourself crazy). Remove the PCB from the back, being careful not to damage the ribbon going to the motor. Remove the metal front cover, remembering that there are usually a few screws hidden behind labels. Less brute force, more patience :o)
Remove the head and magnets by taking out screws. Take out the central screw of from the hub and take off the platter. You might be unlucky and have a platter which sits very low to the backplane. You might need to improvise to jack it up a bit to leave space behind from the leds, or you could combine parts from a couple of drives (as I did) to get the best combination. Another thought is to cut windows for the LEDs in the aluminium backplane, but that seems like a lot of effort!
My replacement platter was cut out of FR4 single sided copper clad board using a 70mm holesaw. A smaller holesaw (19mm I think) was used to cut out the centre, which was then filed to snugly fit the hub of the 2.5" laptop drive. The digits were actually laid out in EAGLE (PCB designer) since it was the only package I had to hand which let you type in rotation angles for characters. This worked OK, but a decent vector drawing package will have more interesting font options. The platter was etched just like a PCB.
It was challenge to fit LEDs and section dividers in the few mm of room behind the platter. My solution was to use SMD components and mount them on flexible kapton copper clad board (awesome stuff) which I picked up on eBay. Just press and peel and etch it as a normal PCB. The finished result, with components on, is only a mm or two in height and can be shaped with scissors and a knife, then glued down in the space under the platter. I made the light dividers from a bit of card cut with a knife and glued to the flexible board.
To index the rotation I used a reflective surface sensor (Osram SFH9202) which detects the passing of a piece of white paper stuck to the back of the platter. I used a strip of matt black insulation tape to give decent contrast on the rim of the platter (FR4 is a bit shiny).
I was a bit worried that the sensor seemed to be getting hot. I am still not convinced this is right, but looking at the data sheet they do dissipate 80mW, and I tried 3 of them and they all did it, so maybe this is right (it hasn't blown up yet!)
The sensor output is quite "analog" and doesn't, by itself, give a nice sharp switching signal. I should have shopped more carefully since Osram do the same sensor with a built in Schmitt trigger output. Still, I had some CD40106 Schmitt trigger inverter chips so I used one of those to give me a clean logic output and it seems to work fine now.
The main board was designed in EAGLE and etched on SRBP using some cheap toner transfer film from a chinese ebay shop. This film is great! it works better than the much more expensive press'n'peel blue and seems less fussy about your ironing technique (i have none). Another first was to etch this with Hydrochloric Acid and Hydrogen Peroxide mixture rather than Ferric Chloride. It is very fast and resulted in less erosion of tracks under the toner, but it also resulted in a nasty lingering chlorine smell pervading the house.
I really enjoy working with SMDs these days, but I used to be terrified of them. Those tiny surface mount components have many advantages: They tend to be cheaper, your boards can be much smaller and need less drilling, and the result is actually really satisfying. I'd say the most important things are magnification (I use a 10x loupe) a decent iron with a small tip (e.g. 0.4 or 0.8mm needle tip), fine solder (e.g. 0.015"), tweezers and a flux pen. Most important of all is practice and expecting it to all go wrong at least a couple of times before you get on a roll with it. Then you'll never look back :)
I'll admit I used solder paste for a couple of the components (the LEDs and their resistors, the two tiny resistor networks, the 16MHz resonator for the Atmega328). I used a hot air tool to reflow the paste. Everything else was done with an iron. The LEDs, resistors and small caps are 0805 size.
I have a small stock of M41T100 realtime clock chips in SOIC8 packages (from an ebay bargain) and I used one of these on this project, with a backup battery.
The MCU is an Atmega328 set up with Arduino bootloader. The code is all Arduino stuff. The Arduino drives the LEDs through a ULN2803 transistor array, since these LEDs are powered in groups of 3 and draw something like 30mA each, which would be too much to drive directly from the Arduino digital output pins.
Last but certainly not least, the hard drive motor is driven by a TDA5144. A word of warning - Don't think you can put power to a hard disk motor and it will spin. These are brushless motors and need electronics to make them work. If you want to try any HDD motor project I recommend you invest in a special IC for it. The Philips TDA5140A and TDA5144 have been perfect for the job in my experience (The TDA5140 - without the A - seems more finicky to get working). You can certainly make your own brushless DC motor controller, but that is a project by itself and I prefer to jump in with the fun stuff :)
Something I was glad I did was to add isolation jumpers to the board so I could power up the Arduino stuff without the motor starting, and vice-versa.
Code and EAGLE files can be found here