When you specify a custom install location for Garageband Content in the Logic 7.2 Installer, two of the instruments are placed at the default location rather than your custom location. These two instruments (Mixed Choir and Vibraphone) appear in a separate folder labeled "GarageBand" when you select an instrument from a channel strip.
To resolve the issue, you can manually move the two instrument files from the default location of /Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments/ to your custom location.
This document will be updated as more information becomes available.