usb disk made bootable

[vygec]

im looking to get a usb hdisk which also needs to be bootable to DOS.
disk needs to be around 250gb.

any suggestions?
Ta.

 

[vygec]

i should have explained myself a bit better.
what im trying to do, is store a number of images and make the same disk bootable to dos. ive done this with some 16gb usb keys without any problems using HP's Utility disk to make it bootable
however with a hdisk the HP utility doesnt work, so im after some software and a hdisk which will work this software.