What is the difference between a startup disk and a bootable disk?
Answer: While these terms are often used synonymously, they have two different meanings. To put it simply, a startup disk must be a bootable disk, but a bootable disk may not necessarily be a startup disk.
When your computer starts up, it boots up from the startup disk. This can be a hard disk, specific disk partition, CD, DVD, or even a USB flash drive. The startup disk is the default disk that the computer starts up from. In Mac OS X, the startup disk can be changed by opening System Preferences, clicking the "Startup Disk" option, and choosing a different startup disk. In Windows, you can change the startup disk by altering the "boot order" in your PC's BIOS. Depending on your machine, you can access the BIOS by holding either F1 or F2 as your computer begins to boot up. Once the BIOS screen appears, you can modify the boot order. For example, you may choose to start up first from the CD-ROM drive, then from the hard drive if no CD is loaded.
So the startup disk is the disk you computer boots up from. This means it must be a bootable disk. A bootable disk (or boot disk) has to be formatted in way your computer can understand (i.e. HFS+ for Macintosh computers and NTFS for Windows computers. It must also have a readable operating system installed, which provides the graphical interface once the computer starts up. This may be a full-blown operating system, or a stripped down version with just enough resources to be functional.