Can't get USB to boot

[billbled]

Windows. 2 different USB drives: Sandisk Cruzer and Lexmark. Just downloaded memtest today. imageUSB.exe to create both. Both showed successful. Both formatted before image creation (I tried NTFS and FAT32). Selecting the correct drive using the boot menu. Both drives in the USB ports (2 different ones tried) directly off the back of the motherboard. Neither boots.
I get black screen, flashing white cursor in upper LH corner for as long as I'd like to leave it. Latest attempt as I type is going on 15 minutes.
Ideas?

[David (PassMark)]

There is no need to format the drive before using ImageUSB as it overwrites the entire content of the drive.

Are the machines you are trying to boot running UEFI BIOS, or old style traditional BIOS?

If they are using UEFI can you send us the log file from MemTest86.
https://www.memtest86.com/tech_debug-logs.html