I am trying to run zerofree on virtualbox .vdi disk. I attached the vdi disk to the storage to another virtual machine - debian desktop. I started the machine and mounted the hard disk with this command: mount -o ro /dev/sdb1 /home/user/mnt. But then if i try to run
zerofree /dev/sdb1/ i get
zerofree: failed to open filesystem /home/user/mnt/
Why i am getting this error?