Not booting from external hard drive

[raylknowledge]

I have usb external hard drive 500GB; how can we boot from it? My bios support USB booting, but it only boot from my USB flash, not my USB hard drive. why???
the usb hard drive will boot if internally on sata. but when it is put on as USB through a sata-usb adapter; it will not boot as USB hard drive. how can I fix this thing???

 

[Dangi]

you should treat it as a pen drive.

First you have to make your HDD bootable as you did with your USB flash, and then while booting press F8 and select USB boot.

 

[raylknowledge]

it was bootable because it was the boot drive while placed as internal ATA.
it is a usb now by means of a ata-usb adapter. is this related the adapter? the usb drive is recognized in windows as USB hard drive, will the bios do the same?