Why do I need to keep all of the old backups?

Question

Yes, I know it is great, but it uses a lot of disk space (my back up is 100 gb, I should a need a 2 tera external hard drive to keep them all!). Is it OK if I manually delete all previous ones (a drag, I know, but might be the only way)

Answer 1

Tif wrote:
Thanks for the replies, and yes, you are right, it is not using the space I thought it was but much less. I am doing what you two suggested here, I am backing up everything from scratch. The thing I don´t understand, though, is ... Why, if when the disk runs out of space TM automatically deletes the oldest one, I can´t do the same manually?
Time Machine's incremental backups back up only the things that have changed since the last backup, which is why the backups can be quite small. New and modified files are added to the Time Machine disk in their entirety; everything else gets a special type of alias - a hard link - that simply points to wherever the real file was last stored (the "wherever" is more a "whenever": it points to the backup that contains the real file). The files aren't moved around; the pointers save all that trouble.
So, let's say you have your original backup, A, and interim backups B and C. You have a new file in backup B, so it doesn't exist in A. When backup C is done, it uses only a pointer to the file in backup B, because you haven't modified the file. What happens when you manually delete backup B? The file disappears and the pointer in C doesn't have anything to point to! Time Machine knows, when it deletes stuff, what's real and what's a pointer, and if it has to change pointers, and so on.
This may not always be an issue if you have a current copy of a file on your drive - in the Now, so to speak. Unless, of course, you modify it and then want to get the previous version (one of the handiest things about Time Machine).
(And Time Machine does erase files when it deletes backups: you can't always get at all the modified versions of a file, for instance, if those dates disappear; and, if you create a file that's stored in backup B, and then you Trash it later, when B is deleted, so is the file and all the pointers to it.)
s.