"What if I don't want to lose the drum sounds that are already on the two user rhythm sets; is it possible to create a rhythm set that's saved to a disk?"

Yes, but it's not saved to disk as an .svd file, but as SYSEX in a dummy song.

First: Your sequencer song memory must be empty. You're essentially creating a new song. Now:

(1) Set up your dream drum kit in TEMP by using the COPY function the way we described above.
(2) See pp. 161f. in your manual.
(3)--> Utility --> 4 Data Transfer
(4) Your source is Rhythm Temp
(5) Your destination is Seq[uencer]
(for now just use the default TRK 1 1-01-000)
(6) --> Execute

Now the SYSEX for the rhythm kit you created in TEMP is in your song. Go to microscope mode and you can see it on track 1.

(7) Name your song (e.g., Drum Kit 1, or MyDrumKit or whatever) and save it to disk using the Song Only option (*not* Song + Sound).

That's it. To retrieve the drum kit, load the song into sequencer memory from disk, play it (you won't hear anything; it will just load the sysex into Rhythm Temp), and look in Rhythm Temp: There's your drum kit.

You can save an infinite number of drum kits (or patches or performances, for that matter) using this method.

The dummy song contains the drum kit. Play the dummy song, the drum kit appears in Rhythm Temp.