Unmount filesystems. Remove a special device or the remote node (rhost:path) from the filesystem tree at the point node. If either special or node are not provided, the appropriate information is taken from the fstab(5) file.
Syntax umount [-fv] special | node umount -a | -A [-fv] [-h host] [-t type] Options -a All the filesystems described in fstab(5) are unmounted. -A All the currently mounted filesystems except the root are unmounted. -f The filesystem is forcibly unmounted. Active special devices continue to work, but all other files return errors if further accesses are attempted. The root filesystem cannot be forcibly unmounted. -h host Only filesystems mounted from the specified host will be unmounted. This option is implies the -A option and, unless oth- erwise specified with the -t option, will only unmount NFS filesystems. -t type Is used to indicate the actions should only be taken on filesys- tems of the specified type. More than one type may be specified in a comma separated list. The list of filesystem types can be prefixed with ``no'' to specify the filesystem types for which action should not be taken. For example, the umount command: umount -a -t nfs,hfs umounts all filesystems of the type NFS and HFS. -v Verbose, additional information is printed out as each filesystem is unmounted. FILES /etc/fstab filesystem table
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umount man page - Apple.com
hdiutil - manipulate disk images
mount - Mount a file system