My laptop doesnt recognise my external hard drive

Hello,

When I plug in my external hard drive into my laptop (Asus, windows 8) it doesnt get picked up. The light comes on on my hard drive but I can find it when I look in my computer. I can see that something is there as I can use the little eject icon where it says I can eject 'expansion'. When I first used my hard drive it worked without any issues and I was able to save some things on there so am not sure why my laptop wont pick it up now - its only a month old. I tried the hard drive on a Mac and it gets picked up there without any issues - therefore is it my laptop?

Please help!!