There are many reasons for booting your ASUS A55A-AH51-RD from a USB drive. For example, if you want to install another operating system as OS X Yosemite, then installing the new OS on an external usbdrive is a good way to install it.This approach will offer you the flexibility of being able to run both operating systems on your computer. However, before you can boot the new operating system from the USB drive, you will need to change the boot order for your ASUS A55A-AH51-RD in the system BIOS.
1, Connect the external USB drive or pen drive to your ASUS A55A-AH51-RD, Press the “Delete” key as it performs the “Power On Self Test” (POST) to enter the BIOS area of the computer firmware. The POST performs a diagnostic of components on the motherboard and does a cursory check of the hard drive, memory and processor. The POST only lasts a few seconds; therefore, press the “Delete” key quickly.
2, Enter the administrative BIOS password if prompted to do so. Once you enter the correct password, you will see the main BIOS screen.
3,Use the right-arrow key to navigate to the “BOOT” menu option at the top of the screen, and press the “Enter” key.
4,Press the down-arrow key until the cursor is on the “Boot Device Priority” label. Hit “Enter,” and press the down-arrow key again to highlight the “1st Boot Device” option.
5,Press the “+” or “-” keys to scroll through the available boot options until the “USB Drive” or “USB-HDD” label is the highlighted option.
6,Press the “F10” key to save changes and exit the BIOS setup program. When ASUS A55A-AH51-RD restarts, wait for the “press any key to boot from external drive” prompt to display, and then press any key on the keyboard to boot your ASUS A55A-AH51-RD from the USB drive.