External HDD won't mount

I use a Toshiba external HDD for Time Machine on my Mac Mini. For some reason it will not mount. The light does come on and I can feel it spinning. It isn’t making lots of noise as a failing drive would and it is only 6 months old. I’ve tried Disk Utility on two Macs and also tried Win 19 and Win 7. I’ve also used two different cables but no good. It does show up but won’t mount at all. I’m not worried about the data as I have Carbon Copy Cloner and a Synology plus an offsite backup, so I can format it. Any suggestions other than destroy it! Thanks