I want to use Time Machine on my Mountain Lion. I have 1TB external HDD I use for storing various stuff.

The thing is I selected the drive for Time Machine to work on but nowhere it is mentioned that it will either format the disk or Time Machine will create a separate folder leaving other stuff on the disk untouched. I am bit skeptical that it might erase my disk. I dont want that to happen.

What will happen if I let Time Machine take backups on that disk? Will it erase everything? Can I use that for time machine without erasing the stuff on it?

1 Answer 1


Time Machine creates a separate folder ('Backups.backupdb') at the root of the external drive in which it stores its backups, leaving all your original data intact.

Time Machine does require your external drive to be formatted as HFS+, by the way. If that is the case, you should have no problems.

  • 1
    I would recommend you use this HDD solely for Time Machine and not keep other files on it at all. Alternatively, I would purchase a new HDD for Time Machine, and continue to use this old HDD to store the files already on it.
    – IconDaemon
    Aug 20, 2012 at 13:18
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    @IconDaemon: Your comment would be more helpful if you also included the reasons for making your recommendation. Aug 20, 2012 at 18:29
  • I feel that something as important as Time Machine needs a new, unused drive which is not used by the file system for any other purpose. Data stored on other HDDs can be backed up to TM in addition to the internal HD, thus giving greater peace of mind. If the TM drive fails it can be replaced and a new TM set created. Personally, I have two 2TB RAID 1 TM disks (OtherWorld Guardian MAXimus) which I switch out weekly and keep one off site.
    – IconDaemon
    Aug 20, 2012 at 19:07
  • Based on my experience with two separate TM backups on the same disk, the trimming part of TM works best and most fair if the disk space TM can use is fairly constant. The concrete case was that one backup grew large causing the other one to start discarding almost all the old backups to regain space. I ended up with losing a lot of history in one backup because the other backup grew too much. Aug 27, 2014 at 10:55

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