Not able to boot from usb?

[c99max99]

I have plop 5 on floppy disk. Plop boots, but when i click usb (Bootable usb is connected) i get "An operating system wasnt found. try disconnecting any drives that dont contain an operating system. Press Ctrl...to restart"

Dell latitude c640 no working os installed

[oldoldschool]

It could be the Linux OS?   you did not say what one you were trying.  I  have used Slacko Linux on an older dell with some success.

[c99max99]

I have tried many OS's some Linux, some other open source projects. None will work. I have used plop and installed some of these same OS's on this PC.

[Elmar]

Hello,

... "An operating system wasnt found. try disconnecting any drives that dont contain an operating system. Press Ctrl...to restart" ...

This is not a message of the Plop Boot Manager. Are you able to make a short video with a smartphone or at least some photos that I can see whats going on on your computer?

When you want to boot Linux, then try as alternative plopKexec.
https://www.plop.at/en/plopkexec.html#download

Best regards
Elmar