我裝左個外置usb harddisk,但唔知佢個位置

I think you didn't assign the drive letter to it or did not partition and format it yet

try this

right click on "My Computer" > select "Manager" > In the Compute Management > choose "Disk Management"

The hard disk should show here, then right click the hard disk, select format, and asign the drive letter you like,

That's it!