MS Support posted an warning regarding STSADM mergeContent DB.
Under certain circumstances, the mergecontentdb Stsadm operation may fail. These circumstances include combinations of significant site collection size, user traffic, and SQL Server load. When the command fails, both the source and destination databases can be corrupted.
If you use mergecontentdb, make sure that you:
• Always back up both the source and target databases before using this operation.
• Set the site collection to be read-only before using mergecontentdb to move it.
• Refrain from cancelling the operation (either on the front-end Web server or the server running SQL Server).
• Run the command during off-peak hours, because mergecontentdb places significant additional load on the server running SQL Server.
Alternatively, consider using Batch Site Manager and the method described here. Because the Batch Site Manager works with site collections by using the Stsadm backup and restore operations with the –url parameter, we recommend that you not use Batch Site Manager for site collections larger than 15 GB.
The Batch Site Manager is included in the SharePoint Administration Toolkit:
• Microsoft SharePoint Administration Toolkit v3.0 x86
• Microsoft SharePoint Administration Toolkit v3.0 x64
Microsoft will release an official KB article for this issue, will keep you posted if the KB is released. Thanks all.