An option to having the two hard drives on the same IDE channel, causing the ATA66 drive to slow the ATA100 drive down to its speed, would be to put the ATA66 drive on the secondary IDE channel, which is most likely where your CD-ROM drive is located. Doing this will bring the ATA66 hard drive down to the ATA33 speed of the CD-ROM drive (if it's relatively new) or to half or even a fourth of that, if it's an older PIO 3 or PIO 4 drive. You've got to decide which is more important: to get the maximum throughput on the ATA100 drive, or the maximum throughput on the ATA66 drive, and then put the drives in the proper places to achieve that.