Just happened to me very weird thing... often, on older motherboards from 2000-2004 -ish era, USB flash drive is not recognized and cannot be booted to (for example Hiren's USB flash drive).

Same thing happened to me on MSI 651M-L motherboard, I've tried every boot option USB-FDD, USB-HDD, USB-CDROM, USB-ZIP, just plain HDD, LS120,ZIP...
nothing helped to boot to USB drive. I've tried enabled USB 2.0 support in BIOS, and disabled. I've tried external USB connector (ASUS MIR adapter via cable connected to motherboard. Nothing helped. I almost gave up and then... not sure why I've tried... enable USB mouse support in BIOS. (it is disabled by default)

and viola... USB Hiren's booted on first try.
I wonder, how many motherboards, I've gave up earlier, would boot up with this trick, because like 1 of 3 motherboard just didn't boot via USB no matter what. Usually with LGA 775 boards, situation is much better, and pretty every board will boot up (like 95% to 99% chance). But from socket A era and socket 478 era, I've noticed, many boards will not boot on USB, even when they have USB booting options in BIOS (usually marked as USB-HDD, USB-FDD etc...)

I was using PS/2 mouse and PS/2 keyboard... so didn't tried it with USB keyboard and mouse, maybe it will not work, when they are connected, and maybe it will work with them. Anyway, that's the reason, why I have options "USB Keyboard support" and "USB Mouse support" disabled in BIOS. They are disabled by default. (in 2000-2004 era, most people were using PS/2 mouse and keyboard) So I didn't bother to change them, when I was using PS/2 keyboard and mouse. Not even have reason to turn it on.

I just don't know, why I've tried it... something was telling me, after trying for 15 minutes to make that USB booting work, before I've tried all possible combinations...and gave up and wanted to quit testing the motherboard.

So I just wanted to share this trick, because from no on, I will use it and always try to turn on USB keyboard and mouse support in BIOS, even with PS/2 stuff connected.
My thoughts are, that maybe MSI motherboard is so stupid, that when USB keyboard and USB mouse is disabled, it won't even run USB ports, like "nothing is inside" It has no sense, I know, because USB flash disk still can be inside, and it has boot options for it. But... that's MSI. Not sure, if this trick helps also on other brands and boards, or it is unique to this one, but I was thinking, everybody should know, this is another little ace in reservoair, that can save you, when USB boot disk is not recognized as booting device. Now I wonder, how many boards during last 4 years that I've tested could be possible to make them bootable via USB stick. Because like I've said like 33% of my old motherboads didn't boot with "USB-HDD/FDD" stuff.