I do not need a solution (just sharing information)
Some of you may have encountered the "The Enterprise Vault installation is currently in maintenance mode and consequently unable to process your request" error and failure of the EV Indexing service to start.
Other threads show that this can be resolved by either removing and reinstalling the Indexing Engine or more simply by renaming the two files called maintenance-mode in the EVIndexing and BIN folders.
This error occurred for me on two storage servers today immediately after replacing the network adapters. So it seems that this service is sensitive to a change in the network card configuration.
Fortunately the quick file rename option resolved the issue in both instances.