Bootable USB's problems

[sendel]

Hi everyone!

 

So my problem is the next:

 

I´ve been playing around with several OSs lately and 've always used two pendrives to boot from them the os´s.

After one crazy attempt of installing OS X in one of my computers i found out that somehow ive messed the usb, so neither of them are recognized as bootable, well they are but i only can get a black screen with a slash spinning or a message saying: boot error.

 

I ve tried to format the usbs several times in different ways without result. they don't boot in ANY computer so i think ive messed with something, although they work fine to simply trasfer files and are recognized in BIOS and by windows and linux.

If someone have any idea of what have i done to messed them up and if it is fixable would be a great help!

 

Thanks!