Hello and thank you for your help !
I've nearly the same situation.
I use a usb key on which is settled "multisystem". This key works on my new computer.
I want to use it on other old computers where it's impossible to boot on usb.
I downloaded the current boot manager plpbt-5.0.11.zip, extracted it to get the iso file plpbt.iso and burned a CD.
I put the key in a usb port of this old computer and plop boot manager in the CD-ROM and I start the computer.
Plop begins ; in the first window which appears there a line usb. If I select it plop blocks : it's impossible to change anything.
What might I do ?
Thank you for your answsers.
Dotao