I have a large photo library (a couple of 100GBs of data) on an external hard drive, but I will need to move it to a different drive.
When moving the files, will time machine try to backup up all the moved files again, so my photo library will be stored twice in the backup set – or is it able to recognize that the "new" files are indeed identical to existing files in its history?
Best Answer
Time Machine works at the file level. Change a single byte in a file, and it backs up again. Move it into a new directory, technically it's a new file, so new backup. There is no way to avoid this in the current implementation. Ideally future enhancements to Time Machine will enable block level backups and some form of de-duplication, which will fix both the above scenarios, but for now you just have to be aware of it. What you can choose to do, if you like, is confirm that the new location is successfully backed u p in the new place, and then manually remove the old location from your Time Machine. This clearly doesn't prevent the need from repeating the backup, but it does at least stop you doubling the storage requirements. Having said that, the old stuff will naturally get recycled off as it's no longer required should the backup drive fill.