The boot order, or the order that your computer looks for bootable media when starting up, is governed by the computer's BIOS system. By default, most computers look at the CD or DVD drive first, followed by the hard disks, and then any other bootable media that may be attached. If you want to make sure that a certain drive will boot, you need to move that drive to the top of the boot order through the BIOS setup utility.
Enter your computer's BIOS setup utility. When you start up the computer, the very first screen that appears will tell you what key to hold down to enter the BIOS setup. F2 is commonly used, but systems vary.
Navigate to the boot menu using the arrow keys. On some systems, this menu is a sub-menu of the advanced settings, so if you do not see it on the main screen, go into the advanced settings and then to the boot menu.
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Use the plus and minus arrows to arrange the order of bootable devices as you desire. With the D drive highlighted, press the plus key until it is at the top of the list.
Save and exit the BIOS setup utility. F10 is the common key for this, but the appropriate key will be listed at the bottom of your screen. This will automatically reboot your computer, and your selected device will be the first one looked at in the boot process.