Enter into the BIOS/setup, expand Drives then select Diskette Drive and set to USB. It should then appear in the boot menu. Let us know how you get on.

I have an old OptiPlex 360 and I can find no way at all to get it to boot to any usb port using any thumb drives or external drives.  It will boot to the DVD drive. I changed every bios setting and returned it to factory settings.  Absolutely fiddled with everything but nothing lets me boot to a usb storage device of any kind.  How is this possible? It will list it as an option but refuses to actually boot to it.  I can boot to a Clonezilla cd but the external drive is unseen.  When booted to Win7 the external drive it totally available.

Either install XP or Install WIN9X dos floppy and run that way.

I actually keep a 3.5 inch usb floppy that is bootable to do this on older dells.

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@skrambam wrote:

I have an old OptiPlex 360 and I can find no way at all to get it to boot to any usb port using any thumb drives or external drives.  It will boot to the DVD drive. I changed every bios setting and returned it to factory settings.  Absolutely fiddled with everything but nothing lets me boot to a usb storage device of any kind.  How is this possible? It will list it as an option but refuses to actually boot to it.  I can boot to a Clonezilla cd but the external drive is unseen.  When booted to Win7 the external drive it totally available.

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I don't think it will even TRY to boot from a flash-drive unless it is "built" properly ... to be bootable.