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When the time machine backup starts, the speed of the backup, as reported by the progress window, seem fine. I have a USB 3 disk and it reports 60GB will be backed up in about an hour. For the first 5 minutes, all goes well, with about 7GB getting backup up.

Then about 5 minutes later, 7.5GB is backed up and the dialog report that it will be 3 hours. After an hour 7.8GB is backed up and it's going to take 13 hours. 1/2 half an hour later 7.8GB is backed up and it's up to 14 hours.

  • I have checked my disk using Disk Utility and it reports no problem.
  • I repaired permissions
  • I stopped the indexing service
  • Restarted the computer
  • I copied a large directory, about 1 GB in size, several hundred files, onto the backup disk using Finder and the copy took about 30 seconds. This is while the backup is hung.
  • Console doesn't report much, but when it slows, it just keeps repeating the same line 'Copied X GB of Y GB. J Items of K Items' over and over

I have actually made several attempts at this, removing the partial backup and trying again, even letting it run overnight. The numbers are different, but I'm getting the same basic result. It hits a certain point, and slows to an imperceptible level. Sometimes at 500Mb, sometimes at 5Gb, now at 7.89Gb (and 16 hour).

I don't think it's the external disk, since it starts with no problems and I can copy files onto it with good speed. I don't think it's the computer's disk, since I have no other performance problems. I have OS X 10.7.5 Lion, all software up to date.

Any suggestions on how to troubleshoot this? Thanks.

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@LauriRanta Yes, they seem to be the same issues, but probably not the same cause, as I am not getting that first error message and, although I do see the second error, it was from hours before I started this latest backup. I also have the differences in the files that the top question has, as related in the comments on that answer. – Steve Mallory Oct 4 '12 at 2:02
Well the backup finished, a little late, at 11:00 this morning. I'm afraid I have no idea what the problem was. – Steve Mallory Oct 4 '12 at 21:44
I am using Mountain Lion and have no issues with time machine backups so far. – Raftalks Oct 22 '12 at 22:16
"I don't think it's the external disk". Try using and internal disk, separate from the one that's being backed up. I'm going to go on a limb and guess that's where the problem lies. The slow rate is probably due to reading / comparing / writing many small files. The burst for a large file may be acceptable on a USB3 external drive, but for anything else it's still slow and tedious. Perhaps the software is poorly written. If possible, try writing the backup to a faster media and then transfer images of the backup (large files, essentially) to the slow USB drive. – Ярослав Рахматуллин Mar 11 '13 at 12:48

As The Russain "Pax" suggested, it's probably the disk firmware, or some such thing. USB 3 is faster, yes, but the maximum speed achieved by the disk is whatever maximum speed they saw in testing. It does NOT mean it is sustainable. Copying many small files is a pain because it is more "meta-operations" to perform before and after the actual copy operation takes place. Such an event will certainly cause your drive to slow down.

Even if you are dealing with only large files, hard drives are weird. Sometimes, the caching firmware on the drive is poorly written, and so when you first plug it in, data can fly by at record speed, but then as the drive continues to be used, it slows down more and more.

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