Intel-based iMac computers are able to boot up using a USB flash drive as the startup disk. A number of maintenance and repair features such as Disk Repair are not functional on the startup drive that is in use. Booting up the iMac from a flash drive allows users to perform maintenance and troubleshooting on the iMac’s hard drive. To boot from a flash drive, users must have a flash drive with Mac OS X installed.
Insert the flash drive into one of the USB ports on the back of the iMac.
Set the flash drive as the startup disk. Go to the “Apple” menu and select “System Preferences.” In the “Startup Disk” window, click on the icon for the flash drive. The next time the computer starts up, it will use the flash drive.
Click the “Restart” button. The iMac will restart and boot from the flash drive.