Not a construction problem but worth saying anyway: If you have any files or definitions that you have created stored on any hard drive, you should BACK UP YOUR FILES REGULARLY. Watch my lips - DO YOUR BACKUPS!
Since this article was first written there have been a few exciting new developments in the area of hard storage. The first being the sheer size of Hard Drive (HDD) available for relatively little money. A 1TB HDD can be purchased for just a little over the cost of a 500GB drive, so there doesn't really seem much point in buying a smaller device, only to find out that as you expand, you need to increase the size of your storage HDD!
The second major development has been the Solid State Drive (SSD). No moving parts and fast access/transfer rates make them about 10 times as fast as HDDs and therefore ideal as a boot drive, holding the operating system, programs and data requiring fast access when you are loading an organ and want fast access to play it.
There are various schools of thought on how many hard drives to use. You can use a single drive for everything or one drive for the operating system alone and one or more others for data, such as samples and definitions as well as configuration files. If you use a single drive then unless you want your PC to be 'minimalistic' buy one that is at least 500Gb capacity. For multi-drive systems I recommend at least a 240GB SSD for the operating system and some VTPO system functions/files. This will be your normal bootable 'C' drive. These days there is little point in building multi-drive RAID arrays. In the rare event your 'C' drive really does become corrupted, it's inconvenient but no big deal to re-install everything, including the OS. You will be backing up your files won't you? You will also need a "storage" drive and a 1TB HDD will be more than adequate.
Don't get caught by the 10000RPM argument. Because they are probably 'Mil Spec', 10000RPM drives are expensive and almost always SATA I. A 7200RPM SATA II HDD has a much faster throughput than a 10000RPM SATA I.
Hard drives come with fixed memory caches which helps to speed up data transfer. Always go for the largest cache that you can afford, this really does help with data transfer speed.
It's also worth remembering that you may well be going to install your PC inside a console and the last thing that you want is to have to do is to scrabble around in the back of the console trying to get the side off your PC because a HDD data cable came loose. It's my opinion that the current design of SATA data cable is an abomination! Remember, organs can produce a lot of bass which will rattle things loose given the chance, so to prevent future problems buy the cables that have a small metal clip to hold them firmly in place. They may cost slightly more than a standard one, but it's a wise investment, but of course if your cables never shake loose, you'll never know, will you?!! You will however have the satisfaction of knowing that you won't be embarassed in the middle of a concert by a SATA cable breakdown.