It sounds like you've got a hard conflict. There's no option in the BIOS to "disable" the Sandisk -- it's usually just silently bumped off when you have a master and slave device detected.
Doublecheck that your hard-drive is explicitly jumpered for "Master", and the CD-Rom is explicitly jumpered for "Slave". If it still doesn't work, you'll have to either find a different CDRom to use, or you'll need to disable the Sandisk in hardware -- either cut it off with a dremel (easier), or desoldering and lifting pins 37 and 39 (if I recall correctly..)
Considering you're using a 40-pin IDE CD-ROM, I'd opt to find another CDROM drive. They're fairly cheap and commonplace these days.