External hard drive can't be mounted. OS X(10.10.4) running huge fsck_hfs process
HUGE PSA for Mac users - if you're external hard drive isn't viewable from anywhere besides disc utility, but you see there is activity on your hard drive (i.e - rapid blinking light), let OS X handle it, it is running a recovery daemon.
You can verify this by simply running Activity Monitor (Finder > Go > Utilities), if you see a process called fsck_hfs, and it's eating up your a good portion of your CPU, this is that recovery daemon!
If you'd like to track the progress of the drive recovery enter this command in Terminal:
tail -f /var/log/fsck_hfs.log
tail -f will update the last 10 lines of this log file whenever it updates.