Mailbox Store will not stay mounted

I just installed Veritas Backup Exec with the Exchange Server agent like on all my machines and ran a backup.  I guess when the machine backs up the mailboxes it dismounts the store.  I started the backup too early so i cancelled it and ever since then the mailbox store keeps dismounting.  I mount the store manually from ESM and two minutes later it dismounts.  What can I do?  Help???????
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   When performing the backup, the store will NOT be dismounted. Check the event logs and post the messages you get there. Only after that we can analyze the issue.

ikm7176Sr. IT ManagerCommented:
space limitations are the first place to look

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ikm7176Sr. IT ManagerCommented:
And the specific problem was ?
RobC56Author Commented:
The database reached the maximum size so i ran a defrag so i could mount the store and then asked users to clean their mailboxes.  It just happened around the same time that I ran a backup.  If I upgrade to Enterprise Server will i have to convert the database to the new one or will the limit of 16GB just be removed?  Thanks for your help.
ikm7176Sr. IT ManagerCommented:
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