Three distinctly different mechanisms exist for loading a disc into a CD/DVD drive: the tray, caddy, and slot. Each one offers some benefits and features. Which type you select has a major impact on your use of the drive because you interact with this mechanism every time you load a disc.

Some drives on the market allow you to insert more than one disc at a time. Some of these use a special cartridge that you fill with discs, much like multidisc CD changers used in automobiles. Newer models are slot-loading, allowing you to push a button to select which internal cartridge slot you want to load with a CD/DVD.

The drive's door opens and you slide in the CD, which the drive mechanism grabs and pulls into place. Typical capacities range from three to six discs or more, and these are available in both SCSI and ATA interfaces.

Tray

Most current SCSI and ATAPI CD/DVD drives use a tray-loading mechanism. This is similar to the mechanism used with a stereo system. Because you don't need to put each disc into a separate caddy, this mechanism is much less expensive overall. However, it also means that you must handle each disc every time you insert or remove it.

Tray loading is less expensive than a caddy system because you don't need a caddy. It is also more convenient, unless you have caddies for all your discs. However, this can make it much more difficult for young children or those who work in harsh environments to use the discs without smudging or damaging them due to excess handling.

The tray loader itself is also subject to damage. The trays can easily break if bumped or if something is dropped on them while they are extended. Also, any contamination you place on the tray or disc is brought right into the drive when the tray is retracted.

Tray-loaded drives should not be used in a harsh environment, such as a commercial or an industrial application. Make sure both the tray and the data surface of the disc are clean whenever you use a tray-loading drive. The tray mechanism also does not hold the disc as securely as the caddy.

If you don't have the disc placed in the tray properly when it retracts, the disc or tray can be damaged. Even a slight misalignment can prevent the drive from reading the disc properly, forcing you to open the tray and reset the disc. Some tray drives can't operate in a vertical (sideways) position because gravity prevents proper loading and operation.

Check to see whether the drive tray has retaining clips that grab the hub of the disc or tabs that fold in or flip over from the outside of the tray to retain the disc. If so, you can run the drive in either a horizontal or vertical position. The main advantage of the tray mechanism over the others is in cost, and that is a big factor. Most drives today use the tray mechanism for handling discs.

Caddy

At one time, the caddy system was used on most high-end CD-ROM drives as well as the early CD-R and DVD-RAM drives. The caddy system requires that you place the disc itself into a special caddy, which is a sealed container with a metal shutter. The caddy has a hinged lid you open to insert the disc, but after that the lid remains shut.

When you insert the caddy containing the disc into the drive, the drive opens a metal shutter on the bottom of the caddy, allowing access to the disc by the laser. When caddy-loaded drives were popular, they were extremely convenient to use if you had a caddy for each drive and extremely inconvenient if you shared a single caddy among all your media.

The caddy was inserted into the drive, much the way you would insert a 3 1/2'' floppy disk. The caddy protected the CD from scratches, contamination, and careless handling. The drawbacks to the caddy system included the expense and the inconvenience of having to put the discs into the caddies.

When DVD-RAM was first introduced, the disc had to remain in a caddy because the recordable surface is delicate. Since then, DVD-RAM drives have been made caddy-less, but especially with double-sided discs the information is at risk every time you handle the disc.

Because of this fragility, as well as the general incompatibility of DVD-RAM with DVD-ROM, I recommend DVD+RW as the best solution for recordable DVD. No caddy is required with DVD+RW, and the format is fully two-way compatible with most recent DVD-Video players and DVD-ROM drives.

The caddy-loading system has declined in popularity because of the convenience of the tray. Today only a few drives on the market use caddies, and they are generally not for mainstream use (most have a SCSI interface, for example).

Slot

Some drives now use a slot-loading mechanism, identical to that used in most automotive CD players. This is very convenient because you just slip the disc into the slot, where the mechanism grabs it and draws it inside. Some drives can load several CDs at a time this way, holding them internally inside the drive and switching discs as access is required.

The primary drawback to this type of mechanism is that if a jam occurs, it can be much more difficult to repair because you might have to remove the drive to free the disc. Another drawback is that slot-loading drives usually can't handle the smaller 80mm discs, card-shaped discs, or other modified disc physical formats or shapes.