When booting up in Win7 drive attempts to access system. External hard drive appears in disk management then drops out. Message indicates not recognised USB. Drive still spins on My Book and light on. Could it be the USB cord? Tried recovery tool but as per above problem.
It could be a bad unit or cord.
Try the steps suggested on the following article:
WD external drive is not assigned a drive letter by Windows or Mac OSX and data on the drive is inaccessible