vista backup

hi folks! would appreciate any help please. after installing vista ultimate not so long ago my first duty was of coarse to backup of my machine, i thought rather than using disks i would use a spare 40gig usbdrive i have instead, after going through the proper procecess seemed to work fine i could see my backup on my drive, then put it in a safe place, took it out 2days l8tr to see how space i had left and for some reason windows cannot find the drive! i can hear the drive see the light on but can windows see NO! i have also tryed the drive on another machine[xp] and still no joy any suggestions would be greatly appreciated........