Why won't an fsck-declared-good volume mount?

Question

I have a CoreStorage logical (virtual), encrypted volume that will not mount, even though fsck_hfs declares it clean. Does anyone know what the deal is here?

Here is the drive information:

/dev/disk4 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *960.2 GB   disk4
   1:                        EFI EFI                     209.7 MB   disk4s1
   2:          Apple_CoreStorage Discover                959.3 GB   disk4s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk4s3
/dev/disk5 (external, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Discover               +959.0 GB   disk5
                                 Logical Volume on disk4s2
                                 9BB5BE72-0024-46D2-A1BB-69AB51247E06
                                 Unlocked Encrypted

where disk5 is the unlocked, but not mounted virtual volume. Here are the results of fsck_hfs on the unlocked virtual volume:

** /dev/rdisk5
   Executing fsck_hfs (version hfs-305.10.1).
** Checking Journaled HFS Plus volume.
   The volume name is Discover
** Checking extents overflow file.
** Checking catalog file.
** Checking multi-linked files.
** Checking catalog hierarchy.
** Checking extended attributes file.
** Checking volume bitmap.
** Checking volume information.
** Trimming unused blocks.
** The volume Discover appears to be OK.

I had put in the password for the encryption, and as I would expect, fsck_hfs seems quite able to read the encrypted data, without it being mounted. This is what happens when I try to mount:

% diskutil mount 9BB5BE72-0024-46D2-A1BB-69AB51247E06
disk5 is a CoreStorage Logical Volume which failed to mount
% diskutil mount disk5                               
disk5 is a CoreStorage Logical Volume which failed to mount

Of course, no clue is given as to why it failed to mount.

I am running the latest El Capitan.

Answer 1

I had same problem... fix/work-around was in Terminal do something like:

mkdir /tmp/3

sudo /sbin/mount_hfs /dev/disk3s3 /tmp/3

df -h  /tmp/3
Filesystem     Size   Used  Avail Capacity iused      ifree %iused  

/dev/disk3s3  464Gi  249Gi  215Gi    54% 3279960 4291687319    0%   /private/tmp/3

BTW, if you have extra ram, you can do faster checks of volumes, esp Time Machine disk, with a command like:

sudo fsck_hfs -f -c 6200m /dev/disk3s3 

** /dev/rdisk3s3 Executing fsck_hfs (version hfs-366.70.1). ** Checking Journaled HFS Plus volume. The volume name is SeaTMimac1012.noindex ** Checking extents overflow file. ** Checking catalog file. ** Checking multi-linked files. ** Checking catalog hierarchy. ** Checking extended attributes file. ** Checking multi-linked directories. ** Checking volume bitmap. ** Checking volume information. ** The volume SeaTMimac1012.noindex appears to be OK.

Tho it was ok, none of these would mount it: DiskUtility (horrible one in 10.12), diskutil mount, etc.