Target information store is taking more disk space than source information store during mail migration. This may lead to the target information store being dismounted, if the storage size was not planned and allocated appropriately.
Exchange Server uses single-instance message storage on a per database basis. With this technique, a message that's sent to multiple mailboxes is stored once, if all the mailboxes are in the same database and copied once to each database that contains other target mailboxes. The recipients receive pointers to the single message.
This Exchange functionality will not be maintained during migration as Quest Mail Agents are logging into the information store on the mailbox by mailbox basis and extracting every single message.
Please plan for the mail database to increase in size by 2 - 3 times during mail migration.
Also please refer to Microsoft Exchange architecture documentation and Solution SOL12558 on our Knowledge base:https://support.quest.com/SUPPORT/index?page=solution&id=SOL12558