UDMA 5 (ATA100)...Why won't it work. HELP!!!!

[Wildfire]

I just got an Abit BX133 MB w/HPT370 RAID controller and 2 brand new 75GXP 30GB IBM ATA100 Harddrives. Yesterday I set up both drives in a stripe set and proceeded to try and install WinME. It would get past scandisk and then lock up. I tried setting up everything again and this time I couldn't even get the drives to format. After a whole day of trying to troubleshoot the problem I finally figured it out. I set both drives to UDMA4 (ATA66) mode and everything worked like a charm. Does anybody know why I can't use UDMA5? I bought these drives specifically for this MB and the ATA100 RAID ability. Is this an incompatability with the HPT370 and the IBM harddrives? I don't think it makes a difference but I have the drives in removable harddrive racks. These racks are completely compatible with ATA66 so I figured they would be ok with ATA100. Am I wrong? Would a Promise Fasttrak ATA100 RAID controller make a difference? Someone please help! Any help would be GREATLY appreciated. Thanks to one and all!