You can keep extra sets by using your CF reader to create a backup of the 'ROLAND' folder from the CF card to your HD. Rename it. Unmount/eject CF card, put it back in your SP (tried this on a 555), power up SP and format card. Now you have a clean card, and a backup of that card's former contents on your computer. Power off your SP, and put the card back in your CF reader. Now, once it mounts on your desktop, copy the renamed backup set onto the card (you can copy as many backup sets as you have on your computer and your CF card will hold). So now you have your CF card with one folder titled 'ROLAND' which is void of samples since you just formatted your card, and a bunch of renamed folders holding samples, seq. etc....
Here is the risk to this method... now that you have all the copied folders on your card... select them and press command + delete to place them in the trash. Now unmount your card WITHOUT EMPTYING THE TRASH.
Your card will function as normal when you put it back in your SP... all the pads are now empty and ready for samples... and you are storing all your other sample sets on the card for later use. Just be careful not to empty the trash with the card mounted on your desktop or all will be lost!
Like I said, I tried this on a Mac... using an SP-555... and it was all good. Hope this is helpful! I copied a bunch of sets to a 2 Gig card using this method and still have a ton of room on it.