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The loading mechanism refers to the mechanical components that are responsible for loading CDs into the CD-ROM drive. There are two different ways that CD-ROM media are normally loaded into the CD-ROM drive.
The most popular loading mechanism used today is the tray. With this system, a plastic tray, driven by gears, holds the CD. When the eject button is pressed the tray slides out of the drive, and the CD is placed upon it. The tray is then loaded back into the drive when the eject button is pressed a second time. Most drives will also respond to a slight "push" on the drive tray by activating the mechanism and retracting the tray.
Many older CD-ROM drives, and many higher-end drives even today, use caddies. These are small carriers made of plastic. A hinge on one side opens up to let you put a disk within the caddy, and a metal cover on the bottom slides out of the way to allow access to the CD by the drive. The caddy is inserted into the CD-ROM drive as a sort of "virtual cartridge". In fact, the CD inside the caddy is pretty similar to the way a 3.5" floppy disk works within its jacket--a media disk inside a plastic protective carrier with a sliding metal access panel. Of course the CD is still removable. Also, the CD caddies are much more solidly built.
Interestingly, some of the other loading mechanisms found on audio CD players seem to be rarely used for CD-ROMs. For example, many car stereos use direct front-loading of the CD; I've never seen this used for CD-ROMs, but I am told that the first Compaq Deskpro 2000 models used this technique. I've also never seen a top-loading CD-ROM drive (but the reason for that is pretty obvious).
Of the two mechanisms, the tray is far more common because it makes for a cheaper drive and also for cheaper use of the media. Most consumer-grade drives use trays for this reason. There are problems with these tray drives however:
Caddies are used on many high-end drives and are a much better mechanism, if you can afford to use them properly. This means that you basically need a caddy for each CD you use on a regular basis. Unfortunately, this can be an expensive proposition, because the stupid things still cost a lot of money--like $5 or more a piece! Frankly, it's kind of ridiculous that they cost this much considering that similar cartridges cost a fraction of this price. It must be just due to the fact that they are sold in such low volume. At any rate, if you have twenty CDs that you use on a regular basis then you are looking at an extra $100 for caddies, which turns most people off to these types of drives in rather short order.
Another possibility is to simply use one or two caddies and then switch the CDs into them as you need them. This is of course an option, but in doing this you lose one of the biggest advantages of the caddy: the reduced handling. You also make using your drive a pain in the rear end, because each time you use a CD you have the added step of swapping the disk into the caddy. (In fact, you have more media handling than you would with a tray drive.) If you can't afford the caddies you are usually better off with a tray drive, unless you especially need the ability to mount the drive vertically.