I have this problem with the Mediabay freezing all the time while browsing for samples. I have a seperate drive which contains my Cubase projects and all of my Samples.
When I browse for samples, the whole Mediabay and Cubase itself freezes up for about 30 seconds. Sometimes it happens a few times when the Mediabay is open.
I use Cubase 6 with a MOTU 24IO audio interface on Windows 7 Pro x64. The Mediabay is about 82GB.