Stop prompting to unlock disk

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.

I strongly advise to use full FileVault 2 encryption instead of using the FileVault 1 wannabe below!

You can mimic the FileVault 1 behavior either with an encrypted sparse bundle image or a second volume encrypted with FileVault 2. Two users are needed though and it's no elegant solution.

A proper backup of your OS X system volume before conducting one of the two how-tos below is strongly advised!

Example how-to with a sparse bundle image:

usera: admin user - usere: user with user folder which should be encrypted

• as usere create an encrypted sparse bundle image - large enough to hold all current and future usere content

• mount the image and create a folder with the name usere and modify it with chmod 744 ...

• log out as usere and log in as usera
• disable "Ignore Ownership on this volume" of the mounted sparse bundle image

• Open Terminal and enter

  su usere
  

• Now - as user usere - copy the content of the home folder of usere to /Volumes/name_of_sparsebundle_volume/usere:

  rsync -aAvX /Users/usere/ /Volumes/name_of_sparsebundle_volume/usere
  

• rename the old home folder of usere:

  mv /Users/usere /Users/userebackup
  

• exit usere - now you are usera again

• link /Volumes/name_of_sparsebundle_volume/usere to /Users:

  sudo ln -s /Volumes/name_of_sparsebundle_volume/usere /Users/usere
  

  You may have to modify the permissions of the soft link with sudo chown usere:staff /Users/usere and sudo chmod 744 /Users/usere.

• Now log out as usera and log in as usere
• Check if everything works

• If everything is fine, remove the folder /Users/userebackup:

  rm -Rd /Users/userebackup
  

Example how-to with FileVault 2:

The method is the same except that you have to encrypt an empty volume (right-click volume -> encrypt volume) instead of creating a sparse bundle image.

You can't log-in to usere directly because neither the encrypted sparse bundle image nor the FileVault 2 volume are mounted while the system boots. You either have to log-in as usera, mount the image/volume, log-out as usera & log-in as usere or you may log-in in console mode, mount and unlock the image/volume and log-in as usere. The latter is untested because I didn't get it to work in my VM.

According to Internet sources encrypted sparse bundle images are prone to (encryption) header corruption. Such a header corruption may render the image un-unlockable!