MacOS – NTFS Disk “failed to mount”

Since El Capitan this is the behavior of Workgroupmanager for local accounts if you enable to manage internal harddrives to be only readable for some users. It doesn't matter which user logs in. If there is only one user managed, the EFI partition will show up on start for all useres. Disabling this in Workgroupmanager again hides the EFI partition, too. Since Workgroupmanager was lastly updated on Mavericks it is still possible to manage e.g. USB shares to be only readable without having to install a OSX Server, as there is no on board solution for managing user accounts more differentiated than with System preferences.

I would try the following since all basic configs/lists/outputs look fine:

• boot to Single User Mode
• /sbin/fsck -fy

• Display free disk space /bin/df

  Example:

  Filesystem  512-blocks        Used  Available Capacity   iused     ifree %iused  Mounted on
  /dev/disk2  6066780416  5644868776  421399640    94% 705672595  52674955   93%   /
  

  Here 94% of the disk space and 93% of the inodes are used.

• If free disk space ~0 /sbin/mount -uw /

• Change directory cd /Users/user_name/path/to/big/file and list files with ls
• Print working directory to be sure pwd
• rm big_file
• Display free disk space again /bin/df
• boot to main volume with exit

You may have to remove some really small files (size < 100 kb) first before you succeed deleting the big file.

If this fails get an external disk, partition and format it (JHFS+) it in Recovery Mode or with a second Mac. Attach it to the broken Mac, boot to single user mode (cmdS while booting), mount the external drive and rsync your user folder and other important files & folders to the external volume. Then erase Macintosh HD and reinstall OS X. You may also rsync your complete system!

Mounting an external drive to /Volumes (even though the boot drive is readable only) should be possible by using:

gpt -r show disk0/1/2 #search for the proper drive because *diskutil* won't work in SU mode
/sbin/mount_hfs -o nosuid -w /dev/disk2s2 /Volumes/ #mount the external drive (e.g. disk2s2) to /Volumes
rsync -aAvX /Users/user_name/ /Volumes/user_name #Backing up the user folder. 

If the above rsync options don't work use something like rsync -aE ... instead.

The root of the external drive is directly in /Volumes (so no volume name in-between like on a normal system: /Volumes/Volume_name_ext_drive/).

After rsync'ing check the content of the "new" user folder.

(Tested in a VM)