Hi, is it 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 or devices.
3. leave it for 10 minutes.
4. Connect back up and reboot.