HDG
I've always heard them called HMG's for hydraulic motor generator.
jorg - two mechs are NOT req'd to do oil checks it is down to company proceedures. I have worked for ones where 2 are required and others where only one can do the job. As long as the NAA are happy with the proceedures in place that is all that matters.
Individual componants may also have an ETOPS restriction. For example a fuel feed pipe of a certain dash number may not be ETOPS compliant IAW the IPC. I know on a certain engine type the spinner is ETOPs critical!!? To prevent problems ETOPS authorised airlines insist all componants aquired are ETOPS compliant even if they are not going onto an ETOPS a/c.
An inoperative power source may not mean that the a/c is no longer ETOPS. It may however reduce the duration fron say 180 to 120 mins. Information like this will be found in the MEL
Hope this additional info helps.