Getting inside your external HDD housing can be troublesome – they are usually heavy devices with plenty of additional plastic in order to protect the disk drive inside.
You’ll need to remove this plastic or rubberised coating, and this is best done with the aid of a flat edged, blunt knife. Once you’ve revealed the screws holding the casings together, you can then get to work on disassembling the housing until you are able to free the HDD.
Before removing the hard disk drive, take a look at the interface it is connected to. While a standard IDE/PATA connector should give you no problems – you can go ahead and disconnect the device before setting it up as a disk drive inside your PC case in the traditional manner – a non-standard connector can potentially bring an end to the project. In many cases HDD manufacturers have made their own interfaces to take advantage of the USB 2.0 interface, resulting in a hard disk drive that you can’t remove and install into your PC.
However don’t rule out using the device entirely. If you have space in your PC for the USB interface as well as the external disk drive and have an internal USB connector on your motherboard or on a PCI card, you should be able to connect the device this way, provided you can also get a suitable molex adaptor to power the device!