We recently carried out a migration to SBS 2011 for a client whose Exchange 2003 database had reached the 75GB limit, which was in part caused by a number of very large mailboxes - in excess of 15GB.

An Exchange mailbox database will dismount every morning at 5:00 when the 75GB limit has been reached. As a temporary fix, before the the migration to a new server, we created a scheduled task to run every day at 05:30 that  re-mounted the database.

To minimise downtime for the customer we started the migration of the mailboxes overnight, starting at 17:30 but we ran into a problem whereby some of the larger mailboxes would not copy over before the next database dismount at 5:00.

To get around this issue, we used the following registry modfication which changes the time Exchange 2003 checks the size of the database. When the database is checked for the first time following the registry modification, if found to be over 75GB, it will only give a warning rather than dismount the database. Therefore, if the value in the registry is set to one hour behind the current time, it will be almost 48 hours before the next dismount. This should be plenty of time to get the mailboxes migrated, but if not, the value can simply be modified again!

To change the time, create a DWORD registry key called "Database Size Check Start Time in Hours From Midnight" in the following location:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Servername>\Private-GUID

Then, specify a decimal value of 1-24 in the value datafield, where 1 is equal to 01:00 or 1 AM and so on. A dismount and remount of the database is required for this change to be active.

The same trick can be applied to the public folder database by adding the same key in the following location:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Servername>\Public-GUID