When ejecting a NTFS flash drive, my system froze and I had to manually power down the machine.
I plugged in the flash drive on another ubuntu machine and got an "unknown error".
When I plugged it back to the initial machine, the same error appeared.
A quick search showed that the command ntfsfix can fix this problem.
sudo ntfsfix /dev/"device name"
For example, if your flash drive is mounted at /dev/sdb, it would be
sudo ntfsfix /dev/sdb
Hope this helps if you run into a similar problem.