How do I prevent an internal partition from mounting at boot
Traditionally, the file you have to edit to prevent automounting is /etc/fstab.

In your case, you would need to add the line shown below to this file.

LABEL=Shared\040user\040data none    hfs   rw,noauto


You can get more information on the /etc/fstab file, by entering the command man fstab.

This also works:

$ diskutil info disk1s4 | grep Volume
   Volume Name:              Shared user data
   Volume UUID:              3D9A5D0C-1307-3178-81FA-9245634906D5
   Volume Total Space:       0 B (0 Bytes) (exactly 0 512-Byte-Units)
   Volume Free Space:        0 B (0 Bytes) (exactly 0 512-Byte-Units)
   Read-Only Volume:         Not applicable (not mounted)
dmaestro$ cat /etc/fstab
UUID=3D9A5D0C-1307-3178-81FA-9245634906D5 /Volumes/Shared\040user\040data hfs rw,noauto


Note that the mount path must be created manually before using Disk Utilities to mount the partition manually, and Disk Utilities will destroy that mount directory when unmounting manually.