I received the error message that the time machine disk could not be repaired and that I should copy the files to another disk as soon as possible. Techtool Pro 7 had been warning me that the disk was running low on space, but it still was about 10% (300 GB of 3 TB). TTP7's Volume structure test shows a validation error in the catalog file. Disk Utility says that counts of thread records , directory item and file hard links are incorrect. It tells me that it can't repair the disk and that I should backup my files are reformat. Unfortunately disk utility will not mount the disk. I'm wondering if the rebuild fails as there is not enough free space. In such case I do have space on a networked Drobo. I was thinking that maybe I could create a disk image there that was large enough and then copy the backup folder to there to repair. But I can't do that since it will not mount .