Why Does My Mac Have Two Hard Drives Listed in Disk Utility?

A Mac computer runs Apple's OS X operating system and comes with a variety of applications you can use to generate files that are saved to the hard drive, such as the TextEdit word processor or GarageBand, a program for making music. When you run Apple's native Disk Utility program, it displays the hard drive or drives mounted on the computer. If you have two hard drives listed, you might have two internal drives, or one internal drive and one external drive.

Every Mac computer comes with an internal hard drive. The OS X operating system is preinstalled on the internal drive so the Mac runs as soon as you take it out of the box and power it up. The internal hard drive also stores preference files for settings you make to applications and the system. When you add new files to the Mac, such as pictures from a camera or attachments from an email message, they are stored on your hard drive. The iMac, Mac mini and MacBook accommodate one internal drive, while the Mac Pro desktop computer has room for four internal drives.

When the Mac's internal hard drive starts running low on space and you want to store more files, you need to either delete some files or add storage space in the form of another internal drive or an external drive. You can get an external drive that runs off of power from an electrical outlet or one that gets its power from the USB cable used to transfer data. Hard drives also connect to the Mac via FireWire cables or over Wi-Fi if they have wireless capability. You can copy files from an internal drive to the external drive, and store new files only on the external drive.

A USB drive is a small device, often the size of a thumb, that has solid-state memory. You can plug it into a USB port on your Mac, and it shows up as an external drive on the desktop and in the Disk Utility window. If you want to use a document on your Mac and then use it on another computer, you can copy it from the hard drive to the USB drive, eject the drive and then insert the drive into the USB port of another computer and open it there.

People sometimes add an internal or external hard drive to their Mac so they can use it to automatically back up the contents of their primary internal drive as a precaution against losing data through accidental deletion or a hard drive crash. After you add the new drive, launch Apple's native Time Machine application. Select the new hard drive as the place to back up your files, and Time Machine starts automatically backing them up. Depending on how many files you have on the first drive, it may take all night to do the initial backup. After that, Time Machine makes incremental backups of only new files and files that have changed in between backups. If you lose files or if a hard drive has a problem, you can launch Time Machine and restore the missing files.