More information would be helpful. Which OS are you using? Do you have the bios set to use IDE or AHCI mode? What is the make/model of your motherboard?
My first assumption is that you have the BIOS set to IDE mode. If so, the drives may have reverted to PIO mode instead of DMA mode. If you are running XP, the fixit button here should correct it.http://support.microsoft.com/kb/817472
You can also change the setting by:
a. Click on start.
b. Right-click on “Computer” and click on “Manage”
c. Click on “Device Manager”
d. Expand IDE ATA/ATAPI controllers
e. Right-click on any label where “Channel” is part of the name – Click on Properties
f. Go to the Advanced Settings tab, under Device Properties check “Enable DMA”