I'm running a late 2012 iMac running 10.9.5 with a similar aged Time capsule connected via ethernet. Been set up like this since I bought it. On Sunday I get an error message saying my back up failed. Error code 999. So I looked and saw that it was just one or two back ups that had failed but they were mostly okay. However since then every back up attempt has failed. It says disk is already in use. As you can see from above, there is plenty of room on the drive. Before it fails I just get the screen above. I figured I'd try repair disk but the drive isn't listed in my disk utility app. Really looking for a non terminal suggestion as I'm not really used to messing in there. Cheers.