The maximum outage due to hardware failure is the time required to repair or replace the hardware plus the time to fully restore the data and bring the Exchange services online. The free space is required to recover a corrupt database. The recovery process involves copying the database file with additional space being used to rebuild indexes. A 50GB information store requires about 10 hours to back up to tape and 10 hours to restore. Any database recovery required after a tape restore would add another 5 to 10 hours to the time required to bring the system back online.
Exchange employs a couple of technologies to conserve space.
First, the contents of the information store are compressed. This means that a user with 8MB of mail on UMC's Showme requires only 2MB to store the same material on Exchange. This ratio is less when the contents of the mail are not simple text messages. The second manner in which Exchange minimizes disk usage is its 'single-copy message store'. The message itself is stored only once.