You can increase the timeout for the File Mounting Service by modifying its App Environment Variable. Look for the property ForensicImageManagerMaxWaitTime for the file mounting service, and you can change its value. It defaults to 300 (seconds), but that can set it to a larger value. Use the query below to change the value.

USE ADD_DB -- Your ADD_DB Name
SET PropertyValue = 'Your#InSeconds'
FROM Enterprise.ApplicationEnvironmentProperty AEP
INNER JOIN Enterprise.ApplicationEnvironmentPropertyName AEPN ON AEP.ApplicationEnvironmentPropertyNameId = AEPN.ApplicationEnvironmentPropertyNameId
WHERE AEP.ApplicationEnvironmentIdentifier = 'YourAppEnvironmentID for the FMS Instance' AND AEPN.PropertyName = 'ForensicImageManagerMaxWaitTime'