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