An expansion bay or bay is an open computer section used for expansion add-ons to your computer, such as a hard drive and CD-ROM drive. On a desktop, drive bays are available in 3.5-inch and 5.25-inches. Pictured is the front of a desktop computer with an empty 3.5" and 5.25" drive bay.
Before installing a new drive (e.g., CD-ROM drive), remove the front drive bay cover from the front bezel to access the drive. When installing a hard drive, the drive bay cover can remain in place because it does not need to be accessible like a disc drive.
What is an external and internal bay?
An external bay is any bay that's accessible to the user without opening the computer. For example, a disc drive would be installed in an external bay because it requires the user to insert a disc into the computer.
An internal bay is hidden and not accessible by the user from the outside of the computer. To access an internal bay, the user would need to open the computer. Computer hard drives that do not need to be removed are installed into internal bays.
Are there bays on a laptop?
No. On all modern laptop computers, there are no bays. However, in the past, there were laptop computers with removable drives. For example, some early computers had bays that could be used to swap between disc drives, diskette drives, and an extra battery. Earlier computers also had PC Card slots that allowed for network cards, modems, tuners, and other devices.