Ultra 66 2nd Hardrive

I installed a second HD. My current HD has Ultra 66 and for awhile I had no problem with 2hard drives. Mu PC automatically detected the 2nd HD. After reininstalling Windows 98 then XP, the 2nd HD causes problems. Sometimes my PC boots successfully, then other times, it will say “detecting operaring system” and then “no operating system found.” I know XP, the Ultra 66 drivers are native, so I don’t know what the problem is? I disconnect the slave HD cable (HD #2 is setup for automatic select) and the PC boots fine.