It depends. If it is a laptop hard disk, and you know the interface (IDE or SATA, there is a difference), you would purchase a 2.5" IDE/SATA Hard Disk Enclosure.
If it is a desktop hard disk, and you know the interface (IDE or SATA, there is a difference), you would purchase a 3.5" IDE/SATA Hard Disk enclosure. Hopefully this helps.
Internal Drive
Most external hard drives can be used as internal ones. Provided that the IDE, SATA, or SCSI interface of the external hard drive is the same as the desktop's system board hard drive interface. Users can remove the USB casing and proceed to plug the data and power cables into it. The external drive, just has an additional component, that's used to hook up with the USB port.
A portable hard drive is just an external hard drive with set memory, for example 320GB, A hard drive enclosure is if you have an internal you wish to use as an external, you simply connect it to the enclosure and use it. However permissions etc have to be overwritten to access the data, so if your not very technical, best just buy an external
The easiest way to make an internal hard drive portable is to purchase what's called a hard drive enclosure. Hard drive enclosures are kits that have everything you need to make your own portable hard drive. Make sure you get a hard drive enclosure that matches the size and interface type as your hard drive, as well as the means of connectivity that you wish. As an example, you can have a 3.5" IDE hard drive enclosure that connects to your computer with a USB cable. Most computer stores have hard drive enclosure. The best place to get one is probably tigerdirect.com. There are many of them available on eBay.
If a hard drive is external to computer then it is easy to move it between computers if required or take it away for safe keeping. If it has a good had drive enclosure then it is more protected from the elements or from accidental damage.
An internal hard drive will generally be faster than an external one.
Yes
There are external and internal hds
It depends on what failed. If the enclosure failed, then you can probably just hook the HD up internally or to another enclosure. If the HD failed, then it's unlikely that you can salvage the information without professional intervention.
I'm going to assume you want to use the old drive as a secondary drive for this answer. Get an external USB hard drive enclosure which is compatible with your "old" drive. CHECK ON THIS, there are several types of hard drive (SCSI, IDE, EIDE, and SATA) and you need to be sure the enclosure and drive are compatible. (Cost: around $30-$40)Put old hard drive in new external case, plug in power source, plug USB into laptop. Plug 'n' play (uPnP) will recognise drive and incorporate it into devices.
There are two types of drives, either internal or external. An external drive would only offer external storage. It is kept internally on the external drive but that is all.
easy to upgrade