-- texto de ejemplo --
~$ mount
udev on /dev type devtmpfs (rw,relatime,size=1027064k,nr_inodes=217162,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=207416k,mode=755)
/dev/disk/by-uuid/af390057-acae-4f90-9e49-e685fb444c8c on / type ext4 (rw,noatime,errors=remount-ro,user_xattr,acl,barrier=1,data=ordered)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,relatime,size=414828k)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /run/shm type tmpfs (rw,nosuid,nodev,relatime,size=414828k)
/dev/sdc1 on /media/Espindarga type ext4 (rw,noatime,user_xattr,acl,barrier=1,data=ordered)
/dev/sda1 on /media/dades type ext4 (rw,noatime,user_xattr,barrier=1,data=ordered)
rpc_pipefs on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw,relatime)
fusectl on /sys/fs/fuse/connections type fusectl (rw,relatime)
binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,nosuid,nodev,noexec,relatime
de ahi sacas lo que corresponda a tu /dev/sdbX y lo desmontas con
~$ su -c umount /dev/sdbX
por supuesto, el dispositivo, no uses la ruta /media/cosas-raras-que-se-inventa-ubuntu-por-que-el-pendrive-esta-petao...