MacOS – Seeking a clean way to mount a directory on one HFS+ drive as a directory on another such drive on same machine

automountfile-sharingfoldersmacosmount

I need to seamlessly mount (and automount, e.g. via fstab) a directory (folder) on one drive to appear at the commandline and GUI level as a directory on another, on the same machine. This is to defeat some "smart" software that detects that symlinks and aliases are not "real" directories and refuses to act on them.

In Linux, this would be trivially easy, with:
mount --bind /path1/source_dir /path2/target_dir
However, Mac OS's version of mount does not support this bind functionality (in any form, including mount -o bind, or mount -B), as far as I can determine. It also appears that none of Macports, Homebrew, or Fink supply a ported version of mount with such an option. Some available more specialized mounting tools don't seem to relate, either (e.g. xmount is for disk images, and djmount is for network volumes).

To be clear, I'm not trying to mount a disk or partition as a volume or as a local directory, nor to create a share to be mounted arbitrarily by various users on various machines. Rather, I want to mount a folder, owned and writable only by a particular user, on an HFS+ disk as if it were a folder on another HFS+ disk, locally, in a way that is effectively transparent, with the same permissions and no security/integrity issues.

A sloppy workaround is to create a CIFS share of the folder to be mounted, and (calling a custom, one-shot variant of smb.conf) limit its permissions in Samba just-so, such that it is not public, and has the same permissions as the target location, exactly the right file and directory mask, etc.; then mount it that way. But this would be inefficient, since it would be using the bletcherous Windows filesharing protocol, plus creating a visible share that shows up as a mountable volume from the SMB server, to anyone who can access this machine though SMB/CIFS (even if they cannot actually mount it). The Mac GUI will probably also represent it as a volume being shared.

Best Answer

To mount a hard drive on a custom mountpoint:

You can do this with diskutil mount and the -mountPoint option. From the man page:

mount [readOnly] [-mountPoint path] device

Mount a single volume. If readOnly is specified, then the file system is mounted read-only, even if the volume's underlying file system and/or device and/or media supports writing; even the super-user may not write to it; this is the same as the rdonly option to mount (8). If a -mountPoint is specified, then that path, rather than the standard path of /Volumes/VolumeName, will be used as the view into the volume file con- tent; a directory at that path must already exist.

From man diskutil, section 'Verbs'.

The syntax to mount to a custom mountpoint is as follows:
diskutil mount -mountPoint /path/to/custom/mountpoint [volume (identifier/UUID/label)]
Bear in mind that /path/to/custom/mountpoint must be a directory, just like with mount, and that your identifier/UUID/label are specific to the volume (i.e. /dev/diskXsY not /dev/diskX). Mounting to a custom mountpoint cannot be done with diskutil mountDisk, and only works with a single volume at a time.

From my answer to How to mount disk by UUID or LABEL in OS X El Capitan.

I've tested this by "mounting" a USB stick on an external hard drive. As an example,

$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh SSD           499.4 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS HDD-1T                  999.8 GB   disk1s2
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *31.6 GB    disk2
   1:               Windows_NTFS USB-32                  31.6 GB    disk2s1

As you can see, /dev/disk2s1, or USB-32 is the name of the USB stick I am going to mount on the hard drive, /dev/disk1s2, or HDD-1T. To do so, make use of the -mountPoint option. But first, make sure there is a landing directory to mount onto:

$ mkdir /Volumes/HDD-1T/mountpoint

And now, to mount:

$ diskutil mount -mountPoint /Volumes/HDD-1T/mountpoint /dev/disk1s2

The mounted disk will appear in the following locations: /Volumes, and /Volumes/HDD-1T/mountpoint as USB-32 and mountpoint respectively.

Be sure to unmount (diskutil umount) your USB-32 equivalent before remounting on your HDD-1T equivalent.

Boot script

The best thing I can come up with, is a script that will run at boot, and will check whether each Apple_HFS TYPE of volume (disk*s*) is mounted. When the volume isn't mounted, try to repair and mount the volume.

Script commands explained

List local HFS volumes

The diskutil list command is used to retrieve all local volumes that contain an Apple HFS file system. For example:

$ diskutil list | grep ":                  Apple_HFS" | awk '{ print $NF }'
disk0s2
disk1s1
disk2s1

Is volume mounted?

The df -lnh command is used to check whether that volume is mounted. When the volume is not mounted, the command returns no (empty) output. And when the volume is mounted, the output is like:

$ df -lnh | grep /dev/disk1s1
/dev/disk1s1   3.6Ti   13Mi  3.6Ti     1%    /Volumes/L

Step by step installation

Create bash script (version #2)

$ sudo nano /Library/Scripts/BootRepairMount.sh

Paste this text into the editor and save it.

#!/bin/bash
TRIES=0
MAXTRIES=60
until diskutil list > /dev/null;do
    TRIES=$(($TRIES+1))
    if [ $TRIES -gt $MAXTRIES ]; then
        exit $?
    fi
    sleep 1
done
TRIES=0
MAXTRIES=3
for OUTPUT in $(diskutil list | grep ':                  Apple_HFS' | awk '{ print $NF }')
do
    if [[ -z $(df -lnh | grep /dev/$OUTPUT) ]]; then
        echo "$OUTPUT is not mounted, repair and mount"
        until diskutil repairVolume $OUTPUT; do
            TRIES=$(($TRIES+1))
            if [ $TRIES -gt $MAXTRIES ]; then
                break
            else
                sleep 1
            fi
        done
        TRIES=0
        until diskutil mount $OUTPUT; do
            TRIES=$(($TRIES+1))
            if [ $TRIES -gt $MAXTRIES ]; then
                break
            else
                sleep 1
            fi
        done
        TRIES=0
    fi
done

Set the permissions of the script for root access only

$ sudo chown -R root:admin /Library/Scripts/BootRepairMount.sh

Make the script readable and executable for all users

$ sudo chmod a=rx /Library/Scripts/BootRepairMount.sh

Make the script admin writeable and executable

$ sudo chmod u=rwx /Library/Scripts/BootRepairMount.sh

Test run the script

$ /Library/Scripts/BootRepairMount.sh

Create a .plist file for launchd to run on boot

$ sudo nano /Library/LaunchDaemons/nl.probackup.bootrepairmount.plist

And paste this text into the editor window.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>nl.probackup.bootrepairmount</string>
<key>ProgramArguments</key>
<array>
<string>/Library/Scripts/BootRepairMount.sh</string>
</array>
<key>UserName</key>
<string>root</string>
<key>UserGroup</key>
<string>wheel</string>
<key>RunAtLoad</key>
<true></true>
<key>Debug</key>
<true></true>
</dict>
</plist>

Change launchd plist permissions

$ sudo chown -R root:wheel /Library/LaunchDaemons/nl.probackup.bootrepairmount.plist

Load the plist into launchd

$ sudo launchctl load /Library/LaunchDaemons/nl.probackup.bootrepairmount.plist

Confirm that the plist is loaded

$ sudo launchctl list | grep .bootrepairmount

If the plist name appears, like:

851 -   nl.probackup.bootrepairmount

it’s installed.

Stop running script on boot

To remove the .plist, in other words stop the script from being run at boot, type the following:

$ sudo launchctl unload -w /Library/LaunchDaemons/nl.probackup.bootrepairmount.plist

All in one line - copy and paste - installation

file=/Library/Scripts/BootRepairMount.sh;{ echo \#\!/bin/bash; echo TRIES=0; echo MAXTRIES=60; echo until\ diskutil\ list\ \>\ /dev/null\;do; echo $'\t'TRIES=\$\(\(\$TRIES+1\)\); echo $'\t'if\ \[\ \$TRIES\ -gt\ \$MAXTRIES\ \]\;\ then; echo $'\t'$'\t'exit\ \$\?; echo $'\t'fi; echo $'\t'sleep\ 1; echo done; echo TRIES=0; echo MAXTRIES=3; echo for\ OUTPUT\ in\ \$\(diskutil\ list\ \|\ grep\ \':\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ Apple_HFS\'\ \|\ awk\ \'\{\ print\ \$NF\ \}\'\); echo do; echo $'\t'if\ \[\[\ -z\ \$\(df\ -lnh\ \|\ grep\ /dev/\$OUTPUT\)\ \]\]\;\ then; echo $'\t'$'\t'echo\ \"\$OUTPUT\ is\ not\ mounted,\ repair\ and\ mount\"; echo $'\t'$'\t'until\ diskutil\ repairVolume\ \$OUTPUT\;\ do; echo $'\t'$'\t'$'\t'TRIES=\$\(\(\$TRIES+1\)\); echo $'\t'$'\t'$'\t'if\ \[\ \$TRIES\ -gt\ \$MAXTRIES\ \]\;\ then; echo $'\t'$'\t'$'\t'$'\t'break; echo $'\t'$'\t'$'\t'else; echo $'\t'$'\t'$'\t'$'\t'sleep\ 1; echo $'\t'$'\t'$'\t'fi; echo $'\t'$'\t'done; echo $'\t'$'\t'TRIES=0; echo $'\t'$'\t'until\ diskutil\ mount\ \$OUTPUT\;\ do; echo $'\t'$'\t'$'\t'TRIES=\$\(\(\$TRIES+1\)\); echo $'\t'$'\t'$'\t'if\ \[\ \$TRIES\ -gt\ \$MAXTRIES\ \]\;\ then; echo $'\t'$'\t'$'\t'$'\t'break; echo $'\t'$'\t'$'\t'else; echo $'\t'$'\t'$'\t'$'\t'sleep\ 1; echo $'\t'$'\t'$'\t'fi; echo $'\t'$'\t'done; echo $'\t'$'\t'TRIES=0; echo $'\t'fi; echo done; } > ~/out_file;cat ~/out_file | sudo tee $file;rm ~/out_file;sudo chown -R root:admin $file;ls -l $file;sudo chmod a=rx $file;sudo chmod u=rwx /$file;file=/Library/LaunchDaemons/nl.probackup.bootrepairmount.plist;sudo launchctl unload -w $file &>/dev/null;{ echo \<\?xml\ version=\"1.0\"\ encoding=\"UTF-8\"\?\>; echo \<\!DOCTYPE\ plist\ PUBLIC\ \"-//Apple//DTD\ PLIST\ 1.0//EN\"\ \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\"\>; echo \<plist\ version=\"1.0\"\>; echo \<dict\>; echo \<key\>Label\</key\>; echo \<string\>nl.probackup.bootrepairmount\</string\>; echo \<key\>ProgramArguments\</key\>; echo \<array\>; echo \<string\>/Library/Scripts/BootRepairMount.sh\</string\>; echo \</array\>; echo \<key\>UserName\</key\>; echo \<string\>root\</string\>; echo \<key\>UserGroup\</key\>; echo \<string\>wheel\</string\>; echo \<key\>RunAtLoad\</key\>; echo \<true\>\</true\>; echo \<key\>Debug\</key\>; echo \<true\>\</true\>; echo \</dict\>; echo \</plist\>; } > ~/out_file;cat ~/out_file | sudo tee /$file;rm ~/out_file;sudo chown -R root:wheel $file;sudo launchctl load -w -F $file;sudo launchctl list | grep .bootrepairmount

MacOS – Can’t mount AFP and SMB shares through Finder but can manually mount via terminal

If this is the same issue as I've seen try this:

Get rid of the user's GlobalPreferences.plist file (it's hidden).

~/Library/Preferences/.GlobalPreferences.plist