External hard drive can't be mounted. OS X(10.10.4) running huge fsck_hfs process

Question

It is sad to say that my external hard drive is currently facing issues.

The hard drive is listed when I run diskutil list (TOSHIBA_EXT):

/dev/disk1
#:                       TYPE NAME                    SIZE       IDENTIFIER
0:      GUID_partition_scheme                        *1.5 TB     disk1
1:                        EFI EFI                     209.7 MB   disk1s1
2:                  Apple_HFS TOSHIBA EXT             1.5 TB     disk1s2

But the hard drive cannot be repair by the GUI Disc Utility, and it cannot be manually mounted with a mount/mount force command because there is a process running on that drive.

I had no idea what this process could've been until I saw a massive process (clocking about 45% of my CPU) running called fsck_hfs.

I piped a ps command into a grep searching for hfs (sudo ps ax | grep hfs) :

847   ??  U     11:57.49  /System/Library/Filesystems/hfs.fs/Contents/Resources/./fsck_hfs 
-y /dev/disk1s2
 999 s000  S+     0:00.00 grep hfs  

So my question is: is OS X automatically trying to recover my hard drive for me? Should I just let it the process run it's course? Why doesn't OS X tell the user what it's doing in this instance? Also, what exactly does fsck do?

EDIT: Here's some more terminal output, this actually seems quite promising:

tail -f /var/log/fsck_hfs.log
/dev/rdisk1s2: fsck_hfs started at Mon Dec 14 12:07:40 2015
/dev/rdisk1s2: /dev/rdisk1s2: Can't open /dev/rdisk1s2: Resource busy
/dev/rdisk1s2: fsck_hfs completed at Mon Dec 14 12:07:40 2015

/dev/rdisk1s2: ** Checking volume bitmap.
/dev/rdisk1s2:    Volume bitmap needs minor repair for orphaned blocks
/dev/rdisk1s2: ** Checking volume information.
/dev/rdisk1s2:    Invalid volume free block count
/dev/rdisk1s2:    (It should be 17331364 instead of 17331357)
/dev/rdisk1s2: ** Repairing volume.

So it finished fsck_hfs and is now repairing the volume?? Why is this process hidden to the regular user?!

Answer 1

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.