How was the two that don't work formated?

Can you please tell us how you solved the problem so as to benefit others who have the same problem.

Very good question. Are large capacity hard drives coming formatted as ExFAT nowadays, perhaps?

Oh sorry, I marked it solved because it looks like a dead end. I can still use the San Disk Glider 16 GB USB flash but will just have to accept that it is recognized as a fixed device.

The other USB flash drives I tried, which BIOS did not display, were formated in either ext2 or 4. I can't remember which. They were originally fat16 or fat32.

The formating matters. If you know of USB drives which will not boot or be recognised please share!

Ok now we're getting somewhere.
Usually the drives are fat not ext2.
What bootable image did you put on drive & how?

For most modern linux and systems a USB is a real physical hard drive. It will not show up as a usb choice. It will show up as a internal hard drive choice.

Only older usb flash might register as usb types. Usually ones below 2G. HP has tools that can help make some bootable. Syslinux sometimes fixes some. Not all usb flash can be made to boot. Almost or maybe even all newer usb flash can be made to boot easily.

Older linux didn't use the scsi (taken from I believe Iomega zip) to access hard drives and won't usually be able to boot to usb of any kind unless updated.

It has been a week or two since I had them (I returned them for a refund). San Disk 8 and 16 GB USB flash. I don't remember the specific names. The one that worked was "glider". The ones which did not work were the "non-glider" ones. I also tried one or two other brands but can't remember the names, though they were also 8 or 16 GB. I did not try PNY because I heard bad things about it. Sorry I can't provide further details. I just don't remember.

So what OS did you try to put on drive & how did you do it.

I used pclos from a live CD. I simply selected "install" and finished the install process. It appeared to install fine on each USB flash but as I mentioned d earlier, only one USB (San Disk Glider 16 GB) booted. None of the others booted even though I performed the same task on them.

Very odd.

I don't think anyone mentioned the bootable flag, yet. One thing to check is that the 'Bootable' flag is set on the bootable partition on each usb drive using the fdisk command. Perhaps you didn't set a bootable flag? That might keep the drive from being listed in a BIOS or the EFI.

By-the-way, I have never gotten a Lexar usb to boot on any computer, but Sandisk and Kingston brands have usually booted. Various USB drives are not bootable or require unique tools to prepare for booting.

Also, don't use USB drives to share data. They are a potential source of viruses or root-kit infection.

Windows 8 certainly is a PITA as far as usb boot options go. I have a san disk Cruzer 8GB and a Kingston Technology 32GB.My Samsung NE350 wants UEFI compliant. Which means, I would have to have a working install of windows 8, complete with sda3 which has all the uefi codes.

I found that very funny.

Lexar! Yes, that was one of the brands that failed for me. I knee it began with the letter "L" but couldn't remember the name.