Can't boot off the USB image

[rocwhite168]

My computers (Dell SX280 PCs or Dell D600 laptop) seem to refuse to boot off the USB disk with non-Windows images written to it (I have also tried OpenSolaris images). I've tried using dd to write the .img files, or using unetbootin to write either .img or .iso images, or using UltraISO to write the iso files to my USB disk, but all the methods failed. But if the image was a Windows boot disk, it did work. Does anyone know what the problem could be? Is it simply because the computers are old enough? Or do I have to do anything special for the FreeBSD images to make the computers boot off a USB device?


Thank you very much!