I struggled with this for a whiler - until I realised the solution.:

First Install "Link shell Extension" Available in 32 & 64 Bit flavours at:

http://schinagl.priv.at/nt/hardlinks...kshellext.html

The following instructions apply to the "Outlook" directory under
"AppData\Local\Microsoft"

1 : Make sure Outlook is closed.

2: MOVE this directories contents to a place on your other Hard drive - For
example a "D:\Mailboxes"

3: DELETE the directory

4: Right click on the directory you now use to store your outlook AppData
contents, and choose 'Pick Link Source'

4: Go to the "AppData\Local\Microsoft" directory, and right click - choose
'Drop Link - As Symbolic Link'

5 Rename this symbolic link (If necessary - in my case my directory was
called mailboxes, so it was necessary) to "Outlook"

YOU ARE DONE!! Outlook will treat this symbolic link as the directory it was
expecting to see.