I have an external Toshiba USB HDD that consists of two partitions: one for Time Machine, and another for iTunes and movies. Since upgrading to Yosemite, the partitions are not mounting properly onto the desktop of my 15" rMBP. The Time Machine Partition mounts, but the icon does not immediately change to the blue Time Machine Icon (I have to re-select that partition as my Time Machine drive). The other partition doesn't appear at all on the desktop. It appears in both the terminal and in Disk Utility, and when I try to mount it with Disk Utility, it says I need to run first aid (although that seems impossible without the partition being mounted). I connected the HDD to both my 2008 MBP a 2012 iMac, and a 2013 rMBP (none of which were running Yosemite) and both partitions mounted instantly, with no issues. Luckily, I was able to backup my data. Why is it that my HDD doesn't seem to mount properly on my rMBP with Yosemite? When I connect the HDD, the console says: diskarbitartiond: unable to mount /dev/disk2s2 (status code 0x00000047). Any feedback would be greatly appreciated, as I need to access the files on that drive on my rMBP.