Even when you have ample free space on your Time Machine backup drive, it’s wise to ensure that it is well used. After all, what you want there are your backups, with the minimum of overhead. Every gigabyte of storage lost to backups means that, as a backup, it will last less time, and contain fewer backups.

It used to be that you could exclude your Time Machine backups from Spotlight’s indexing. Not only does this save a great deal of time with the mdworker background service chundering through your backup drive making indexes, not only does it save all the resulting disk activity, but it also saves a lot of space. And the space which it saves inevitably grows as your backups grow.

Try adding your Time Machine backup folder to the files excluded from Spotlight indexing, though, and you will be told that you cannot. If you add the whole volume to the Privacy list, then Spotlight will ignore your wish, as regards the backup folder. Not that it will admit to that, of course.

Apple explains this by stating:
If you add a Time Machine backup disk to the privacy list, you will continue to see messages that Spotlight is indexing your backup disk. This indexing is necessary for Time Machine to function properly and can’t be disabled. Spotlight does exclude from searches any items you store on your backup disk that are not part of a Time Machine backup.

This change appears to have happened at some time around 2011 or 2012, possibly in macOS 10.7 Lion. From occasional comments seen online, some really determined souls have tinkered at the command line and stopped indexing on their backup volumes, only to discover that Time Machine backups stop too, so it looks fairly deep and irreversible.

The reason that this came to my attention is that I have been running a third-party Spotlight-based search utility when writing an article recently. One thing this app did was force a complete rebuild of the Spotlight indexes on my external RAID drive, which is used almost entirely for Time Machine backups, and was added to Spotlight’s Privacy exclusions.

I was most surprised when the RAID was thrown into intense activity, and I am sure that it will remain throbbing away for the next day or three. With over 2.5 TB of backups going back over the last four years, there’s an awful lot of metadata to trawl through, which means an awful lot of space for its indexes. But I don’t want to search it using Spotlight, as there are too many files there…

Thanks for taking away that option, Apple.