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Suggested Boot/Master Volume Partitioning for Large SSD

Last updated 2014-12-12 - Send Feedback
Related: Mac Pro, Other World Computing, SSD, storage

OWC Mercury Aura SSD for Mac Pro: 1TB about $899, 2TB about $1479.

Please see Terminology: Volume vs Drive.

How to use all that fast SSD space?

With a 2TB SSD (or 1TB), it can be useful to separate the startup volume (Boot) from the main bulk of user generated data (Master, as in “master copies”).

If something goes wonky with the boot volume, it’s possible to install over it, or even to erase it without affecting the Master volume. Moreover, it’s a useful organization approach. See Separate Data from System and Applications.

As shown, a 2TB OWC Mercury Aura SSD for 2013 Mac Pro has been partitioned into a 200GB Boot volume and a 1720GB Master volume using Apple’s Disk Utility. [Disk Utility description as “External” is erroneous, as is the drive name of Accelsior].

Continues below.


With SSDs smaller than 1TB, partitioning this way balkanizes the space (wasting it), and is thus not recommended.

Unlike a hard drive, partitioning an SDD does not affect performance; it is organizational/structural only.

However, it is wise to choose appropriate capacity partitions so as to not waste capacity on one or the other. Since most user data will go on Master, the Boot drive can be relatively small—MPG uses 120GB as a boot volume.

To partition the internal SSD, boot off any external volume, e.g.:

  1. Clone the internal startup volume to any external volume; reboot off the external volume (set the startup volume).
  2. Partition the internal SSD as desired.
  3. Clone from the external boot volume to the internal boot volume.
  4. Set the startup volume to the internal boot volume, reboot.

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Organization and backup

The system software and applications should all go onto Boot. Defaults for Mail, Calendar and similar should in most cases remain on Boot (in the user’s home folder).

User-generated data such as word processing, spreadsheets, Lightroom Catalogs, etc should go onto Master. Large files (videos, raw image files, etc) do not benefit from an SSD and are best dealt with by using an external volume, the OWC Thunderbay 4 being an excellent choice for those needing high capacity.


Backup both Boot and Master, because various important things will still be on Boot. Plus it is very useful to have a bootable clone.

Backup is straightforward with cloning; see Using Cloning as a Backup Strategy:

  • Clone Boot to BootClone
  • Clone Master to MasterClone

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