[Couldn't find more appropriate forum] My MBP was running 10.8.5 (Mountain Lion) and had Time Machine backups going back to September last year. I re-formatted the drive and did a clean install of 10.11.3 (El Capitan) in April. I gave it the same machine name as it had before (simply "macbook"). I expected that even though I had used the same machine name, there would be a new unique identifier of some sort associated with it, and that OS X and Time Machine would behave as if this was a new (never before seen) machine. I was surprised when I turned on Time Machine to use my Time Capsule, it started backup up to the existing "sparsebundle" file (i.e., "macbook.sparsebundle") which had been created while running Mountain Lion. I believe it did a full backup, as one would expect, but it seemed to be preserving the old backup history. I thought, "pretty neat!" However, when I recently tried to restore a file from before my El Capitan install, the Time Machine interface shows the backup dates all the way back to September, but anything prior to April is "greyed out" and inaccessible. The interface won't "zoom back" to those dates, even to list the files. Are these old backups accessible in some way? If so, I might keep them, although I don't really feel a need to. If not, I wonder why Time Machine didn't either delete them or start a fresh sparsebundle file. That's what I expected to happen. To me this seems like a (minor) bug in Time Machine -- that it added on to the old backups yet doesn't allow access to them. I wonder if it can even delete these old backups when it comes time to thin them out.