An external hard drive is a data storage device that attaches to a computer with a standardized cable, typically a USB cable. Similar to an internal hard drive, it stores data on a series of rotating platters and you can format it for a number of different computer platforms, including Mac OS, Windows and UNIX, among others. For the most part, Macs can read a Windows-formatted external drive, but the reverse is not the case.
For the most part, hard drives are cross-platform, meaning you can use the same drive on most computers. However, in some cases, you must reformat the drive for the computer to recognize it, such when connecting a Mac OS-formatted drive to a Windows PC or a UNIX machine.
A Windows external hard drive will work with a Mac, provided that you're using a connection type that's also supported by the Mac, such as a USB or FireWire model. However, all of the files that are on the disk may not be readable on your Mac. Also note that if there are any programs on your PC external hard drive, you can't open them on your Mac.
The one exception to PC-formatted external drives working with Macs is when you attempt to use a Windows-formatted drive with Time Machine, Apple's integrated data backup utility. Time Machine prompts you to specify a target disk, but will not accept it if it isn't Mac formatted.
If you have to reformat your PC external drive, copy any vital data off before proceeding. Then attach it to your Mac, launch Disk Utility (found in "Utilities" within "Applications") and highlight the external disk on the list in the left portion of the window. Select "Erase" from the list at the top of the window and then select either "Mac OS Extended (Journaled)" or "MS-DOS (FAT)" from the Format pull down menu and click on "Erase."
Understand that a Mac-formatted drive works only one way. A Windows computer won't be able to read or write to a Mac-formatted drive but will recognize that a drive is attached and prompt you to either eject it or format it to be recognized in Windows. In all instances, back up any vital files on the external drive before attempting to attach it to another computer.
- Photo Credit Thomas Northcut/Photodisc/Getty Images Jupiterimages/Photos.com/Getty Images George Doyle & Ciaran Griffin/Stockbyte/Getty Images