JSI's web site has the following tip. It's for NT, but may bear on the problems getting install to work cleanly on Adaptec 2940 based systems. By the way, I think the current potato boot floppies have fixed the problem anyway.
2018 ยป Error when booting the NT 4.0 or Windows 2000 CD with an Adaptec 2490UW? If your Adaptec 2940 BIOS is setup to allow booting a CD-ROM, you may experience: BIOS 1.22 CD-ROM became Drive C: ,Hard Drive became Drive D: Error: "MISSING OPERATING SYSTEM" BIOS 1.23 CD-ROM does not become DRIVE C: Error: "BOOT: Memory overflow error Please insert another disk." To fix the problem, upgrade the BIOS to 1.25 or later.
