You can see bellow:
root# run show system storage
Filesystem Size Used Avail Capacity Mounted on
/dev/ad0s1a 432M 344M 83M 81% /
devfs 1.0K 1.0K 0B 100% /dev
devfs 1.0K 1.0K 0B 100% /dev/
/dev/md0 354M 354M 0B 100% /junos
/cf 432M 344M 83M 81% /junos/cf
devfs 1.0K 1.0K 0B 100% /junos/dev/
procfs 4.0K 4.0K 0B 100% /proc
/dev/bo0s1e 48M 17K 48M 0% /config
/dev/md1 168M 12M 142M 8% /mfs
/cf/var/jail 432M 344M 83M 81% /jail/var
devfs 1.0K 1.0K 0B 100% /jail/dev
/dev/md2 39M 4.0K 36M 0% /mfs/var/run/utm
/dev/da0 1.8G 168M 1.6G 11% /cf/var/tmp/usb