Disk Partition Not Mounted

Whatever happened to my partition icon on the Desktop? Disk Utility can see it (grayed out) and has verified it. Can somebody suggest steps I can take to get the icon back on the desktop. It has reappeared magically in the Finder/Go Menu, and Time Machine can once again backup to it. Other programs don't see it: When I ask for a recent item in Mariner Calc, for instance, an error message comes back (File Not Found). Yet I can open files manually on the partition and those files (created in Mariner Calc) will call the originating program,

Any help will be appreciated. Thanks

Your Mac model and year? Version of OS X you're running? Which external hard drive are you using?