When Time Machine appeared on the scene, I loved it. Still do, but I have now learned the hard way why one should never use a Time Machine backup disk for anything other than Time Machine.In my innocence, I thought, "Well, this is my backup disk. It's huge and far bigger than Time Machine will ever need. In addition to letting Time Machine store its stuff there, why don't I also use it to store all those very-rarely-used documents and movies too?"Wrong.Nothing bad happened for a couple of years. Then, inevitably, some really cheap and really small terabyte drives appeared on the market. I bought one just for Time Machine, and let the old drive lie fallow for a while. Then, just last week, I decided to delete the old Time Machine backup files. I dragged the backup folder to the trash, and then emptied the trash. Or so I thought.It turns out that Time Machine plays a subtle hand. Those files it stores in the backup folder are not ordinary files -- they have been changed in ways that I still do not fully understand. The Finder may or may not be able to delete them (usually not). The file count, as it supposedly deletes, can go negative -- a very disconcerting event. The directory structure may or may not survive the Finder's bungled attempts to delete. It can take hours or days to determine that something bad happened, at which time it can be too late to recover. And do not even think about dragging the backup files out of the Trash -- this results in a different kind of hellfire and damnation.I was lucky -- my directory was reparable, with effort. Having repaired the file structure, I was ultimately able to delete the recalcitrant files with the aid of the Terminal. I navigated to the offending drive with the Terminal, and then issued the magical incantation "sudo rm -rf .Trashes". It worked, but it took all night.On delving into online discussions of these arcane matters -- which I should have done much earlier -- I learned that many people are not so lucky.Moral: DO NOT EVER TRY TO DELETE A TIME MACHINE BACKUP WITH THE FINDER.In fact, one should follow online advice and dedicate one volume to Time Machine, and never use it for anything else. Then deleting a backup folder is easy: just reformat the drive, and Presto! it's done.If, however, you are like me and don't follow advice unless you can see the reason behind it, then you may someday need to delete a Time Machine backup folder from a volume that also contains other large and important files. In this case, there are two alternative courses of action, neither of which use the gormless Finder.1. Enter Time Machine and use its ability to delete backup files and folders. It will do this without fuss or Finder-like hysterics.2. Open a Terminal session, and use the Apple-provided utility tmutil. Here is a link to the manual pages for tmutil:developer.apple.com/library/mac/#documentation/Darwin/Refere...If, like me, you have arrived at the point where it is already too late for sane and sensible steps like the two above, then you can try the same magical incantation that worked for me. But be very careful: any Unix incantation that starts out with "sudo rm" is very potent black magic, so potent that in the hands of the unwary it can destroy entire volumes of information in a flash.LorenP.S. Here is a useful page which explains how Time Machine works: pondini.org/TM/Works.html
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