Get PID of process that prevent from unmounting a USB drive

$ fuser -m /var/run/media/mycomputer/mythumbdrive/
/run/media/mycomputer/mythumbdrive: 1583m

See what that process is

$ ps ax|grep 1583
1583  ?      S   0:02  /usr/bin/python /usr/lib/python2.7/site-packages/rabbitvcs/services/checkerservice.pyc
6859  pts/0  S+  0:00  grep --color=auto 1583

Oh, it’s rabbitvcs. Now kill it

$ kill -9 1583

Now I can remove the USB safely