In SBS2008 and 2011, there's a wizard allowing you to move the WSUS repository to another partition/disk. However, it does not move the state database, which, over time, can grow a bit large. Luckily, one could manually move the database by stopping WSUS, detaching the database, move the files, reattaching it, and finally, starting WSUS again.

First of, stop the required services (WSUS and IIS Admin Service)

net stop "update services"
net stop w3svc

Next, detach the database:

sqlcmd -E -S np:\\.\pipe\MSSQL$MICROSOFT##SSEE\sql\query -Q "sp_detach_db 'SUSDB'"

Move the SUSDB.mdf and SUSDB_log.ldf to their new location. Then, reattach the database:

sqlcmd -E -S np:\\.\pipe\MSSQL$MICROSOFT##SSEE\sql\query -Q "sp_attach_db @dbname=N'SUSDB',\
       @filename1=N'D:\WSUS\SUSDB\SUSDB.mdf',\
       @filename2=D'E:\WSUS\SUSDB\SUSDB_log.ldf'"

Finally, start the services again:

net start "update services"
net start w3svc