The Diablo 3 installer will not allow you install the game to a folder on an A: or B: drive. This can cause a problem for folks who run their OS off of a small SSD at C and who place most of their game files on a larger HDD at B.
There is a workaround for this which goes as follows:
1.) Temporarily change your B: drive letter
For Windows 7 users
-Open your start menu.
-In the search box type Disk Management .
-From within Disk Management select your B: Drive and right click it and choose Change Disk Letter or Paths on the context menu.
-Change your drive letter to something other than A: or B:. I chose X:.
**After doing this a number of programs will act screwy and many of your shortcuts will temporarily break**
2.) Install Diablo 3 to the location of your choice within the X: drive using the installer
3.) Go back to Disk Management and change your drive letter back to B:
Hopefully Bliz will get this straightened out as the workaround is a chore.