incremental backups of exchange mail stores

Hi,  our exchange mail strore is big, 275GB at last full backup.  There is an option to do incrmental backups and flush non comitted logs... but when I did this last night and looked at the BE12 log I  saw this:

 

 

Family Name: "Media created 8/12/2008 10:27:21 AM"
Backup of "\\marge.emtex.com\Microsoft Information Store\First Storage Group "
Backup set #32 on storage media #1
Backup set description: "Backup 00029"
Backup Method: Incremental - Logs (flush committed logs)
Microsoft Exchange Server Agent: Started

 

Backup started on 8/13/2008 at 10:11:50 PM.

 

Backup Set Detail Information

The consistency check of the snapshot for the Microsoft Exchange transaction log Logs was successful.

Backup completed on 8/13/2008 at 10:12:32 PM.

 

Backup Set Summary

Backed up 0 Exchange Server stores
Processed 173066738 bytes in  42 seconds.
Throughput rate: 236 MB/min
Compression Type: Hardware

This backup set may not contain any data.

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 What  does this mean?  I'm sure some data changed in the store since last backup.

 

I am jusing icnrementals and full backups using Archive bits.

 

I really can't backup the whole store everynight as it would eat through an awfull lot of tape.

Can anyone tell me how they backup large MS Exchange info stores?

 

thanks

 

BIll