When creating a new mailbox database with the Exchange 2010 Management Console you may have noticed that the database and log paths are automatically populated.
This path comes from the value of the DataPath attribute on the Exchange Server object.
[PS] C:\Admin>Get-ExchangeServer | fl name, *path* Name : EXCH2010-TEST DataPath : C:\Program Files\Microsoft\Exchange Server\V14\Mailbox
Unfortunately this attribute is not exposed to modification via the Set-ExchangeServer cmdlet. However like all attributes it is stored in Active Directory. In this case it is the msExchDataPath attribute on the Exchange Server object, viewable in ADSIEdit.msc.
You can edit the attribute in ADSIEdit.msc and it will take effect immediately. The next time you run the New Mailbox Database wizard it will pre-populate the paths with the new value plus the name of the new database.
[PS] C:\Admin>Get-ExchangeServer | fl name, *path* Name : EXCH2010-TEST DataPath : C:\ExchData