Slow SCSI performance

I have a Dell Precision 410 Workstation, with dual 650 P3, 256 MB ram, W2K Pro SP1. I have a 40G IDE and a 4G SCSI hard drive.

The SCSI drive is connected to the built in Adaptec AIC-7890 Ultra2/Wide LVD (Adaptec 2940 U2W-equivalent). The driveis an older Western Digital Enterprise Single Ended Ultra Wide drive.

I have used the drive in a Mac, with a UW controller, and the benchmarks show it is able to transfer both read and write at the 10-15 MB/s range. Using NTBench I am only seeing 3 MB/s write and 6 MB/s read. I was expecting more.

In addition, another machine running NT4 SP6 with 4 LVD 16 gig drive running RAID 5 on an Adaptec raid card, are only able to achieve 1 to 2 MB/s write with caching turned on.

What am I missing?