Why is my external hard drive greyed out in finder and on desktop?

I've got a Drobo attached to my Imac and recently the icon has greyed out on the desktop and in the finder. When you try and open it nothing happens- no error, nothing. Finder just wont look at it. I've downloaded path finder and that can see it no problem. I can even drag a folder shortcut from pathfinder to the mac finder and I can get into the drive that way...has anyone experinced this before? Even better- wnyone know how to fix it? Many thanks!Greyed out in Finder- no eject option either!

Greyed out icon on the desktop