VirtualBox: grain table inconsistency

VMware has a tool called vmware-vdiskmanager located in "/Applications/VMware Fusion.app/Contents/Library/" and symlinked to /usr/local/bin when installed. You can use this tool to repair VirtualBox VMDK disks as well. It saved me a couple of times already.

Installation

a. You can install vmware-fusion

brew cask install vmware-fusion

b. Or you can download the vmware-vdiskmanager utility directly from Attachments section at the bottom of this page:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1023856

Usage

vmware-vdiskmanager -R <your_disk>.vmdk

Invoking without parameters gives help usage:

Usage: vmware-vdiskmanager OPTIONS <disk-name> | <mount-point>
Offline disk manipulation utility
...
     -R                   : check a sparse virtual disk for consistency and attempt
                            to repair any errors.
...

Can't really help, but I found what the grain tables are here: http://www.vmware.com/support/developer/vddk/VirtualDiskAPIprogramming.pdf (page 16)

Like the user mailq says, looks like your virtual drive is broken. Maybe check the vmdkck tool (on this page http://datto.org/projects/vmdk-tools ) to double check it's broken?

Your virtual hard drive is broken. Grain tables are some internal stuff of virtualization environments.

Googling for the exact term reveals that there are not many possibilities to recover: https://forums.virtualbox.org/viewtopic.php?f=6&t=40049