A boot device is any device or drive that has the required boot files that allows the computer to start. For example, a floppy disk drive, CD-ROM drive, DVD drive, and USB jump drive are all considered bootable devices. However, unless the necessary boot files are stored on the drive, diskette, or disc the computer will not be able to boot from that device.
The most commonly used boot device or boot drive is the hard drive. When an operating system such as Microsoft Windows installed onto the hard drive it copies the boot files and drivers required to load windows on the computer. When installing a new operating system, troubleshooting the computer, or booting from another drive such as the CD-ROM drive, a bootable disc or setup disc is inserted into the drive. If the boot sequence is setup properly in CMOS setup and the bootable disc is inserted the computer it will load from the disc instead of the hard drive.
No boot devices available
If the computer hard drive is not working, has not yet been setup, or there are no other bootable devices you will receive an error such as "no bootable device" or "no boot devices available" when the computer boots up. This error is an indication that the computer looked at all available devices, but did not see any drive with boot files. If the computer was working before getting this error message and no any new hardware has been added, it's very likely that your hard drive has failed.
However, before assuming the hard drive has failed make sure no cables (especially USB cables) are connected to the computer and that all diskettes, CD's, and any other USB devices are disconnected from the computer.
Inaccessible boot device
If the computer has detected a boot device, but is unable to boot from that device you'll receive an error such as "inaccessible boot device" when the computer is booting. This can be an indication of a corrupt boot device or a diskette, disc, or USB drive attached that the computer that is not bootable. Remove any diskettes and discs from the computer and disconnect any USB drives, MP3 players, or cameras connected to the computer.