When StableBit Scanner is installed the Easy2Boot batch file MAKE_E2B_USB_DRIVE (run as admin).cmd fails when trying to format a USB drive. The script runs RMPrepUSB for the job, and it claims the drive is not accessible and suggests closing explorer windows or applications that are using the drive.
The only thing that actually worked was removing the scanner. I suppose there's some low level conflict somewhere with how it occupies or "tags" USB drives. I just wanted to tip about it.
I checked beforehand if the Scanner was doing anything, but it had not started scanning it yet at all.