The non-Time Machine partition appears to mount correctly and can be used as usual (although as mentioned above it cannot be unmounted).It sounds like my hard drive has failed (or is in the process of failing) - the only things that I makes me second guess myself and make me wonder if the disk is salvageable are: I tried erasing the disk but that also did not work (error message: Couldn’t unmount disk.) Back up as many of your files as possible, reformat the disk, and restore your backed-up files.). Via Disk Utility, I am unable to either mount the Time Machine partition (I get the generic not able to mount partition error message) and I am also unable to run Repair disk on it (error message: Disk Utility can’t repair this disk. If I hit the eject button in Finder, the partition name greys out and is not clickable but does not go away. I can't seem to either eject the disk or unmount the partition that is mountable.reading and writing (I've already copied all my data off it) When I unplug the drive and replug it back in, only the non-Time Machine partition shows up in Finder. On boot-up, both partitions do not show up in the Finder at all Just today I noticed that the hard drive is not mounting correctly anymore I have a WD external HDD which is plugged into my MacBook Air more or less all the time it has two partitions one of which is my Time Machine disk.