If it is the usb external means check whether the USB ports are working properly or not ...

if you are connecting and trying from the front usb then once you try inserting in the backside port of the cpu ..

normally the external drives are only detected by the windows it is cannot be bootable at the startup before starting of the operating system ...

so you must be using minimum windows XP and if you are using it then connect it after booting into the windows ..

then it will detect a new hardware and you can start using the external drive ...

if there is any manufacture cd install the software if any ..

if you still persist the same problem then check it in other system ...

if you still persist the same problem then there may be some manufacture defect just take replacement as you will be having the warranty ....

regards