Hello, sorry to be so late to answer, but if those drives are USB or Firewire...
reset FW bus, same goes for USB reset...
Reset the Firewire bus
If your Firewire or USB isn't recognizing any device. A solution which has worked for some whose hard drive became invisible in 10.4 was simply to follow these four steps to reset the Firewire/USB bus:
1. Shut the machine down.
2. UNPLUG the power lead to the computer and any firewire/USB drive.
3. leave it for 10 minutes.
4. Connect back up and reboot.