Let me elaborate a bit more on some of this.
1) Possible cause: The boot order is set incorrectly.
Make sure the USB drive was attached when he went into the BIOS settings. The BIOS options will be different depending on whether the drive is seen at boot or not. Disable anything that says boot USB and confirm the external drive doesn't show in the Boot sequence (usually listed a few layers deep in the list of hard disk drives [HDD]).
2) Possible cause: The BIOS changes the boot order back after you change it.
Basically, try #1 again just to make sure.
Then, do a backup to a different drive before attempting any of these steps because this will put your data at risk.
Some BIOSs will change back to booting the USB device if you leave it disconnect for a boot. Removing the "Active" status from the partition on the drive will usually fix that.
So you can reformat the external drive or use a utility called Diskpart.
Note that DiskPart is a very dangerous utility than can easily erase all data on your computer with an incorrect key stroke, so it should be used with caution and only by experienced computer experts that know the value of a backup and have several.
You can see if the drive has an "Active" partition by looking in Disk Management. It will say "Healthy, Active" in the white area of the external drive.
I'd recommend just using Disk Management to reformat it, and give the drive a secondary partition this time.
3) Possible cause: The BIOS mistakes the USB drive for a Mouse or keyboard and hangs attempting to negotiate with it.
Disable any "Legacy USB" setting in the BIOS. The drawback is you won't have access to a USB keyboard or Mouse until USB drivers are loaded in your operating system (not usually a problem in Windows, but could be a problem for Boot CDs). So, with that in mind, look into reloading your motherboard's USB drivers. You would need search for updated USB drivers at your computer manufacturer or motherboard manufacturer website.
If you have a Dell PC, go to Dell's site and search for USB drivers and/or something similar. If you have an HP, do the same at HP's website.
If your computer is not an "out of the box" ready-made computer like one of these name brands, check your motherboard's user manual for its specifications and brand. Then, go to that manufacturer's website to look for updated USB drivers.
Search for updated USB drivers at your computer manufacturer or motherboard manufacturer website.If you have a Dell PC, go to Dell's site and search for USB drivers and/or something similar. If you have an HP, do the same at HP's website.If your computer is not an "out of the box" ready-made computer like one of these name brands, check your motherboard's user manual for its specifications and brand. Then, go to that manufacturer's website to look for updated USB drivers.
If these steps fail or are unavailable, try making Windows search for updated drivers. Please follow the instructions in Document ID: 201235.
If that fails:
a) Right-click on each individual item under the subheading Universal Serial Bus controllers.
b) Select the Uninstall option on each device.
c) Then reboot your computer.
4) Possible cause: The BIOS just can't negotiate with USB drives (usually seen on drives above 500GB) and hangs.
Updating the BIOS or putting a USB hub between the BIOS and the drive can help, but in some cases there isn't much that can be done short of disconnecting the drive during boot.
