Apple - Unable to mount Time Machine partition
I had the same problem and resolved it like this.
- Unplugged the drive
- Turned off Time Machine in Pref
- Rebooted the Mac
- Plugged the drive back in
- The Time Machine partition mounted
I would guess it's most likely a logic corruption/error and not a physical corruption of the drive since the other partition seems unharmed. This has a chance of happening if you are writing/reading data and suddenly unplug your drive. Can't be 100% certain so if I were you I would definitely try to backup the other partition to another drive in case it fails as well.
(If you could post a picture of how the drive appears in Disk Utility it would be great!)
WD has a 3(?) year warranty (you can check the warranty status of your drive here) on HDD's that suddenly fail which would cover a manufacturing error.
Before going through the tedious RMA process I would try to:
1) Unplug the drive
2) Reboot the Mac (without the drive connected)
3) Connect the drive
4) Open Disk Utility again and try to repair/fix the partition
5) Run sudo diskutil verifyVolume /dev/diskN (where N is the volume name not the disk name) to try and see if the file system structure can be read at all.
6) Depending on the outcome of the command (it should return something like "the volume X appears to be ok" if it's fine) I would try to repair it with sudo diskutil repairVolume /dev/diskN
7) As a last resort, run sudo diskutil eraseDisk {Format} {NewName} /dev/diskN (replace {Format} and {NewName} with your preferred file system and new name for the drive) which will try to erase the disk and apply a new file system. This will remove your previous partitions and you will lose the data on the disk.
If I'm being stubborn I would try to find a good disk repair software and run that to diagnose the drive before doing the RMA-process.