I'm assuming this is an external disk connected directly to your computer. In that case, it's likely to be a loose or faulty cable connecting the drive to your machine. When it wiggles a little, the connection gets lost. (That also could cause corruption in your Time Machine backups when it happens in the middle of a backup... better repair that hard drive using Disk Utility!) You may be able to make it work more reliably by changing cables, or by leaving the machine untouched on a table or desk while the backups is running so it doesn't get bumped.
If the cable isn't the problem, it's possible the disk itself (or the enclosure it's in) is faulty, or that the port on your computer that you're connecting the drive to is damaged.
I too had this problem... twice. First let me say that I'm using Western Digital Caviar Black HD's I salvaged from my Dell craptop as they weren't that old. I installed them (two) into external USB 3 enclosures I got from BestBuy, Dynex brand.
The first time I had the problem I had to exchange the enclosure because it was bad. Same deal with the second time, only I don't get to exchange it (out of warranty)... nor would I want to at this point. Moral of the story for me is "buy better enclosures".
Oh, I tried making the drive read/write and it did not help.