Today my backup was delayed for an hour or two for one simple reason: I forgot to exclude the backup volume from being indexed by Spotlight.
Thank you Steve Jobs for insanely great design— and insanely presumptuous design also: I don’t want any backup drive I attach to be indexed by Spotlight. Why should attaching an external drive invoke any kind of indexing at all? The most frequent use of an external drive for many users is for backup. And backup drives do not need to be indexed.
The nagging problem that never goes away is that even if you opt-out of indexing a drive, as soon as you erase it, your choice is forgotten, and you’ll have to do it all over again.
If you see the rudely interruptive daemon 'mds' and/or 'mdworker' running, that’s the process pair that indexes your drives for the Spotlight functionality. If mds decides to index your big backup drive while you’re backing-up, a 1 hour backup might take 3-4 hours, or more, as the drive heads seek like crazy trying to serve two programs: one copying files to the drive, and one reading it like crazy to index it. With most hard drives you’ll hear the drive seek noise.
In my view, mds should never index a drive that is in heavy use. It’s a bug based on sloppy thought (or no though) about how computers are actually used. It is a no-brainer solution to have indexing deferred until such time as the drive activity is quiescent. Better yet, make indexing of external drives an opt-in process.