i have an quicksilver with 3 drives in it...NOT the MDD....
you will need a nice screwdriver and a lot of patience!
the hard drive will need to be put in the zip drive bay mount. so here's what you need to do...
1. buy the hard drive
2. open the case
3. disconnect the IDE and power supply to the optical drive
4. unscrew all screws holding the optical drive harness in place
5. carefully remove the harness with the drive in it
6. make sure your jumpers are set to "cable select"
7. slide the hard drive in the zip drive slot, ports facing the back of course
8. you will need to hold the drive in all the way back and connect the cables (notice that there is an extra end on each of the optical drive's cables...use the last spot for the hard drive)
9. connect the optical drive's cables
10. screw the harness back into place
11. close case and reboot
12. open disk utility and partition the drive
13. enjoy your third hard drive