External HDD partitions not mounting properly on rMBP?

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.

I can't answer why Yosemite is not properly mounting the second partition on your Toshiba drive. What I can tell you is that we do not advise using a single drive for both Time Machine and storing data or anything else. As inexpensive as external hard drives are, it's advisable to keep one drive dedicated to Time Machine and another for data or whatever.

You might wish to run Disk Utility verify and repair on the second partition of the Toshiba drive to see what it says.

I actually solved the issue. I had modified some .kext files because of some WIFI troubles, and it appears as though the file that was responsible for mounting FAT32 drives was changed. I reinstalled Yosemite and everything works smoothly now.

Glad you solved it. Modifying kext files can get you into trouble real quick but I'll assume you know what you're doing. Another bit of advice that you can consider: Keeping data on a FAT-32 partition is not a good idea. FAT is a non journaling file system that is prone to data errors and loss.

Thanks for the tip! I use FAT32 because it's readable by my game consoles when I want to play video files. Now that I have an HDMI input, that may not be necessary. I really appreciate your help!