I'm really glad that the PSR-9000 is working out well for you. I love mine, too. Hard drives for the PSR-9000 can be tricky. Toshiba drives seem to work well. Remember, you MUST have a 2.5 inch drive (that's what fits in most notebook computers).
You can buy a 2.5" Toshiba 6 gig drive online at http://spartantech.com
(that's where I got mine) for $152.00 plus shipping. However, you must have access to a notebook computer so you can partition and format the drive. If you do not have access to a notebook computer, then I would simply buy one from your music dealer to avoid problems.
Partitioning the drive: Boot the notebook computer with a Windows98 startup disk (boot disk), then type FDISK to start the partition software. Follow on screen prompts, its not hard. You'll need to make 3 partitions on a 6 gig drive. Then use the Format command to format each partition. Incidentally, be sure to label each partition when formatting the drives - the labels will appear on the PSR-9000's screen.