I've had this one!
What happens is your SBLive drivers are assigning your card one IRQ then windows is assigning it to another IRQ. Then windows gets "confused" and drops your plextor.
This is what I did:
I made a floppy with only the boot files for DOS (like you do when you setup a bios flash disk).
When the system gets done booting from the floppy you'll be able to see a list that the bios puts up. The list shows the devices in your system and what IRQ's they are using. You can see this list without the floppy, you just need to be fast on the PAUSE button.
For starters make the SBLive have it's own IRQ, that should take care of the disapearing plextor. Then to add more stability to your system get the latest drivers for the SBLive.