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When the computer starts then it tries to figure out which device to boot from (Hard-disk, cdrom, dvd, usb, flash, floppy, etc.). If no device is found or if a faulty device, then one of the following messages can be shown:

Invalid boot device

No boot device found

No boot device available

Unable to find boot device. Select proper boot device or insert boot media

There can be different causes for this problem:

• Disk a have been left in a removable drive:
  Usually caused by having inserted a disk in the floppy drive or the CD-ROM drive or attaching an USB flash drive. Just remove the disk from the drive, and it should be solved. More Info MS KB Q812492
• Hard disk controller doesn't recognize the right boot drive:
  One should verify that hard disk cables are properly attached and that the hard disk is being detected properly by the system.
  If having added a new hard disk, then one should make sure that the disk controller doesn't attempt to boot from the new hard disk.
  If having reset the motherboard BIOS configuration, then one should check that the primary boot device is configured properly.
• Boot partition is not active partition:
  If having been working with partition tools (Like Disk Management or Fdisk), then one might have removed the Active Partition setting from the boot partition.
  • Vista - Use Windows Recovery Environment and perform a "Startup Repair".
  • Win2k / XP / Win2k3 - Use Recovery Console and run fixboot
  • Win9x / DOS - Use a partition configuration tool like Fdisk to set the active boot partition.
  More Info MS KB Q315261

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