I recently purchased iMac with Yosemite (10.10.3).
Having 1 TB HDD, I created a partition named "MacWork". For last 3 days this partition is missing in finder and in DiskUtility a greyed name (disk0s4) is coming.
I did verify this disk but it showed error saying can't fix the disk pls save your data and format.
Its a nightmare, I have some of my running project's data on this disk but can't access. I reached to apple support but they said the disk got corrupted.
Don't think disk warrior can rewrite a directory it can't see. If It won't mount, it can't help.
There are other recover programs that will grab whatever files it can.
Hard drives die and develop bad sectors.
Backing up is a must for any can't afford to loose data.
Its the down side to all this advanced stuff we do.
Its not reliable.
Mac Pro, macbook pro, Imacs (i7); Canon 5D Mark III/70D, Panasonic AG-HPX170/AG-HPX250P, Canon HV40, Sony Z7U/VX2000/PD170; FCP 6 certified; FCP X write professionally for a variety of media; teach video production in L.A.