Running XP. Then check their " current transfer rates" and not their maximum manufactured rates.
Right click my computer > properties > device manager > IDE ATA / ATAPIE controllers > pick your channel > advanced settings.
These are the transfer settings for your hard drives. Expect around Ultra MA 4 for the ATA100 and either Ultra DMA2 or pio4 for the ATA 33 drive.
The problem of one drive slowing the other was combated by dual FiFo ( first in first out ), I think?
You can test this by going to pcpitstop and puting the machine through it's tests. Make sure you defrag the drives before testing;-)