I want to install some applications on an external SSD attached via a USB 3.0 interface.

As I know from the apple site, I can move my iTunes and photo library to an external drive. (support apple link)

I own the 128 GB version of the MacBook Pro series, so I would like to outsource some disk space-intensive applications on an external SSD like the libraries, too.

Can I install applications on an external drive?

If it is possible to install applications on an external SSD, will the OS load the data from there to the internal SSD to process or is it going to load the data directly to the RAM ?

  1. Yes. Some applications won't allow you to install them on drives that don't contain an OS X installation, however there are often ways to do it manually post-installation if that is the configuration you wish.

  2. Some applications will indeed do some writing to the internal SSD, since presumably that's where OS X is installed. The Library folders for example contain configuration settings and application data for most of the applications on your computer. Although you can trick an application into writing to your external SSD by moving its data/configuration directory onto your external drive, then making a symbolic link back to your Library folder. It's not guaranteed to work, but it generally works just fine.

  3. An application whose data is entirely contained on the external SSD won't have any reason to access the internal one. The data would load directly into RAM, never once touching the internal SSD.

  • Addendum on point #3 -- if your home directory and OS install is on the internal drive, the application may use disk cache and application-specific setting storage on the internal drive. Really: it's complicated. – Ian C. Jun 7 '15 at 17:25
  • 1) Yes. Some applications may require that there be an operating system present on the the external drive, however.

  • 2/3) Entirely depend upon whether you are booting from the external SSD (in which case an OS must be installed). If so, then all R/W operations will occur on the external drive. No I/O activity will occur on the internal drive unless you subsequently choose to mount it.

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