Transport endpoint is not connected

I have exactly the same problem. I haven't found a solution anywhere, but I have been able to fix it without rebooting by simply unmounting and remounting the mountpoint.

For your system the commands would be:

fusermount -uz /data
mount /data

The -z forces the unmount, which solved the need to reboot for me. You may need to do this as sudo depending on your setup. You may encounter the below error if the command does not have the required elevated permissions:

fusermount: entry for /data not found in /etc/mtab

I'm using Ubuntu 14.04 LTS, with the current version of mhddfs.

This typically is caused by the mount directory being left mounted due to a crash of your filesystem. Go to the parent directory of the mount point and enter fusermount -u YOUR_MNT_DIR.

If this doesn't do the trick, do sudo umount -l YOUR_MNT_DIR.