Knowledge level: If you've cracked open a case before, you should be able to follow what I say. If not, well, you can try at least. I doubt you would be able to follow, as I'm not quite sure how to simplify it too much more.
So, hard drive, simple, right? Bigger is better, right?
Not quite. There are several hard drive factors that people should look at when picking one out.
First, however, to clear up a few terms. Hot swappable means, literally, it can be swapped while "hot" - or while power is on to the computer. So a hot swappable hard drive can be unplugged while the computer is on, whereas one that isn't hot swappable cannot be (without problems at least). A controller is, basically, a controller for the drive - it has the ports for the cables for the hard drive or other devices.
Interface - Bar none, the single most important thing to decide on.
- Parallel ATA - Most computers have Parallel ATA (PATA or PIDE for short) controllers, cables, hard drives, and CD/DVD drives. The cables typically look like long flat ribbon cables (although occasionally cut up and put in to a round plastic tube or net) with 40 pins. It comes in three main speed types today - ATA/66, ATA/100, or ATA/133. Most hard drives (along with CD/DVD drives) are ATA/100 - what this means is that it uses [parallel] ATA at 100 MB/s maximum throughput. However, hard drives very rarely even approach this maximum - having a PATA hard drive transfer at 50 MB/s is considered to be very good. PATA drives are, compared to most other varieties, slow. They also have a really short cord length (18" maximum) and should be internal only as they are not hot swappable.
- SCSI - If you have to ask what SCSI is, you don't have it. Mainly used for higher end servers, SCSI devices are technically superior in every way save cost effectiveness to their PATA cousins. I won't say more on it since it isn't really necessary.
- Serial ATA - The newer standard, Serial ATA (SATA for short) controllers are on pretty much all newer motherboards. They first started to appear around three years ago in the common computer and are here to stay for awhile. SATA runs at speeds of 150 or 300 MB/s (as of this writing at least). They are hot swappable, have much smaller (although they can be longer - up to one meter in length) cables, and are in general just nicer drives. However, if your system does not have an SATA controller, don't bother trying to get one - just grab a PATA drive. Also, SATA drives often times require their own power connector - be sure you have it available, although you can buy an adapter pretty easily for it. Finally, Microsoft Windows 2000 and XP both have issues installing on SATA drives - more on that later.
- USB - Ah, the mainstay of external devices. Chances are, you have at least 2 USB ports on your computer. Chances are, at least one is in use. USB hard drives have two standards - USB1.1 and USB2.0 Since the difference between USB1.1 and USB2.0 could take up an entire post, I'll explain the difference some other time, so lets assume USB2.0 (which all current hard drives are now). The entire USB bus has a max speed of around 60 MB/s - so no matter what, USB2.0 drives are the slowest of the crop. In general, you won't make it too much past 15 MB/s, accounting for all other USB devices. Also, USB hard drives are basically just normal PATA hard drives with a USB Enclosure, like this one. It is typically cheaper to just buy the hard drive and enclosure separate. Finally, most large MP3 players (like large iPods) double as a portable hard drive, and work pretty much in the same way.
- Firewire - Oddly shaped ports, firewire ports are similar to USB ports in a lot of ways - external, hot swappable. However, firewire is fast. VERY fast. In fact, firewire can be even faster than a SATA interface (although the hard drive probably isn't any faster). Most of the other points dealing with USB (such as the enclosure) also deal with Firewire. However, beware - most computers do NOT come with a firewire port (notable exceptions are computers from Sony and Apple).
Short summary: PATA, SATA, USB, Firewire. Choose one. SATA would be my choice for all internal hard drives (if the computer supports it), and either USB or Firewire for external ones, depending on use.
So, now you have the interface, time to go for the largest drive possible, right? WRONG!
Cache. Caching is, basically, the idea that if you read a bit of data from one part of the hard drive, the bit immediately after it is the most likely thing you are going to want next. So, hard drives have cache (2 MB, 8 MB, or 16 MB usually) to try to store up information that is likely to be accessed next. Because of caching, hard drives can appear to be MUCH faster than normal - the bigger the cache is, the more data you can grab in one shot if the guess is right. The downside is, a lot of time that guess is wrong. Bigger cache is better, but you really don't need anything above 8 MB unless you have a really big hard drive. Avoid 2 MB cache drives unless price is your primary concern.
Okay, okay, cache and interface, can I go grab a big drive yet? NO!
Spindle speed. That number measured in RPM. Most desktop hard drives today are 7200 RPM, with some slower ones being 5400 RPM. On laptops, most hard drives are 5400 RPM, with some slower ones being 4000 RPM. However, faster does not always mean better. Basically, this number is how fast it will rotate - a 7200 RPM drive has the ability to rotate the drive 7,200 times in one minute. However, if the drive is slow to access otherwise, a faster spinning drive doesn't really help all that much.
In fact, faster spinning drives can be a problem. On laptops especially, faster spinning drives require more energy, meaning less battery life for laptops and higher electricity bills all around. Also, faster spinning drives create more noise. Luckily, most drives today have an "acoustic mode" - a fancy looking phrase that just means that they slow down the drive to make it quieter.
Also, the really fast drives are 10k RPM or faster - those tend to be high performance and low capacity drives. They also cost a heck of a lot more, and if you are really concerned about how fast an operating system will load, this is for you.
So, in general, ignore the number unless you are concerned about super speed (which means go for the higher numbers) or for power consumption or noise (which means go for smaller numbers).
There are a couple more factors too.
Access Times. There are a number of ways to measure access times - consecutive access, random access, spin-up access, whatever. Basically, companies will try to use numbers to their advantage, so if you are looking at access times, compare the same values across - don't compare one company's consecutive access time to another company's random access time. Otherwise, lower access time = better.
If you want to be more picky, if you are looking at a drive that has a lot of static data on it (say... a drive for MP3s), look for smaller consecutive access times (and probably higher cache). If you are looking for a drive that has a lot of dynamic data (like operating system hard drives), look for drives which have random access times on the low side (which probably means a faster spindle speed). If you are running a laptop that disables hard drives quite often for power reasons, you'd probably be more concerned with spin-up access times.
Form Factor. What good is a hard drive if it won't fit in your case/enclosure/laptop? Make sure if you buy a hard drive that you make sure it fits - for desktops, a 3.5" form factor is the standard and used pretty much everywhere. Smaller for laptops (with different sized laptops using different form factors - 2.5" is the normal size), and just make sure your enclosure is the right size for the drive.
Warranty. Unlike most computer parts, hard drives die very, VERY often. The average hard drive lasts around 3-4 years, which is why most warranties are 3 years long. Some warranties, however, are only 6 months to a year. Other warranties, especially for higher end drives, are 5 years or even lifetime. Make sure the warranty on the drive is longer than the length of time you are planning on using the drive as a primary drive. If this is meant for a server, a 5 year warranty is good. Otherwise, 3 years is good enough - just expect to replace the drive shortly afterwards.
Capacity. In general, an operating system drive's capacity isn't all that important, if that's the only thing you have on it. Since the smallest HD (as of this date) whose price is actually worth getting is an 80 GB drive, it'll be perfectly fine for an OS. For data storage drives though, the bigger you get, the better. Right now, sweet spots on prices (per GB) are at the 160, 200, and 250 GB marks - anything bigger will have a premium on price.
Oh, and one last thing. Windows 2000, XP, and 2003 do not like installing to non-PATA drives (this includes SCSI, SATA, USB, and Firewire). In general, they require a textmode (also known as a realmode) driver for the controller. Most times, this will be a floppy diskette included with the motherboard, but personally, I recommend slipstreaming. You can find the directions to do so typically on the website of the manufacturer of your motherboard, but I'll cover it in more detail in another post. In general though, slipstreaming means that you are putting the driver (or any other updated component) on the Windows CD itself, eliminating the need for that floppy diskette.