They don't need to be inside anymore!
For a long time, external hard disks were more of a curiosity than something you could realistically add to your PC. If you've been in need of high performance from an external disk or disk array, you've had to spend a fortune on external SCSI, Fibre Channel or some other enterprise-level connection medium. High capacity has gone along with that high price requirement. So for realistic money, in the home or single user price range, you've been stuck with, erm, nothing really.
A couple of things have conspired to change that in recent years.
The first is the massive drop in hard disk prices. You can get large capacity and density for very little money, especially when it comes to regular 3.5" form factor disks. The cost per GB ratio is way less than £1/GB now, which means that stuffing loads of capacity into your PC, either internally or externally, won't break the bank.
The advent of FireWire and USB are the second. They are low cost to implement and the bus protocols are flexible enough to attach IDE or SCSI disks to a bridge, since supporting their command set using the bus as a transport is possible without much difficulty at all.
The move in USB spec to USB2.0 and the appearance of FireWire ports on an increasing number of motherboards and laptops, along with low-cost PCI cards that implement each bus, means that the busses are now fast enough and available enough to use successfully with hard disks of a decent speed and size.
So we've got low cost per GB, massive capacity disks and the means to cheaply attach them to a modern PC. Of course the high-end has remained one step ahead. You can't add multi-terrabyte, 200MB/sec+ arrays to your PC using USB2.0. You still need something a little more exotic. But for our purposes of whacking an arse load of extra storage onto our PCs, on the cheap, it's now very possible.
To that end, I've grabbed four external hard disks in different sizes, from fairly small to oh-my-god-that's-a-big-hard-disk-you've-got-there big, using both FireWire and USB2.0 connections, to see what's available these days. There should be something for everyone.
The obvious restrictions of each interface being used means that my focus isn't necessarily on performance - although I will evaluate it - but rather on things like cost, packaging and capacity.
Three from Maxtor and one from Freecom, let's see what they bring to the table.