USB booting :: making bootability stick

I have several pendrives, different sizes and makers, which I have got to boot dsl.
But after some time of disuse, they won't boot any more.
Running syslinux on them makes them bootable again, but only temporarily.
Anyone seen this or have any ideas ?
tia

They won't boot at all or do you get to the boot screen and then knoppix can't be found?

they aren't seen as bootable and the machine goes to the HD to boot

Try doing the following:

Enable USB keyboard emulation in your BIOS
Disable Fast Boot in your BIOS

Try a "Soft Reboot" IE: CTL-ALT-DEL

and see if the "bad" drives can boot up.

thanx for the suggestions
the keyboard thing seems to help on my more current machine - the other one is too old for such fancy bios tweaks!

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