Carbon Copy Cloner is taking 2.5-3 hrs to work its mojo these days. I suspect this has something to do with the switch to Lion (the same process took 20 mins on SL.)

In any case, is it advisable to continue to use the Mac while CCC is backing up the drive? It's a one HDD iMac.

1 Answer 1


The Carbon Copy Cloner FAQ states:

Yes and no, it really depends. Performance will be affected during the clone (especially the first one) as CCC reads the entire source volume and writes to the destination volume. If your work is "disk bound" -- that is your applications are reading or writing to either the source or destination, then you'll notice a performance hit. If you're just reading email or writing a Pages document, then you probably won't notice the performance hit.

Affecting the accuracy of the backup task is something else that should be considered. Typically it's OK to work from the source volume while you're copying it, with the understanding that if CCC copied a file, then you open it, make changes, save it, then CCC completes the backup task, the modified version of your document is not backed up (this time around). Typically that's no big deal, the modifications will get backed up the next time the backup task runs. More importantly, though, if you're working with large files (mounted disk image, Entourage email database, VMWare/Parallels container) during the backup operation, it is possible that those large files could be modified while CCC is backing up that file. This won't affect the source file, but there's a good chance that the backup version of that file will be corrupt. For this reason it is a good idea to stop using applications that may be modifying large files.

TL;DR: there is performance hit depending on the intensity of file I/O you produce; files you work on will not be backed up; large files may get corrupted.

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    I only read the TL;DR part, but good answer! Oct 25, 2011 at 13:48
  • Excellent answer, cheers. I wonder if the iTunes library database would be counted as a 'large file' for these purposes? I've been doing a lot of writes to iTunes of late and this is my chief concern. It's good to know that I can browse around on the web and not have to worry tho'. Anyone got any ideas re: a ~200 GB iTunes library if I were to make, say, 1 GB of writes to it while the back-up was happening?
    – boehj
    Oct 25, 2011 at 15:55
  • That seems to warrant another question :). Quick answer: depends on what it is mainly made of – there is no monolithic “iTunes database”, just lots of media files of varying sizes (movies are big, songs are small).
    – kopischke
    Oct 25, 2011 at 16:01
  • What I mean to say is, every time anything is changed (e.g. I rate a song) it seems the whole library is changed. This could be 'iTunes Music Library.xml' and/or 'iTunes Library.itl' I believe. It just seems that something has changed in Lion whereby a once very quick process now takes some hours. Cheers for your answer.
    – boehj
    Oct 25, 2011 at 20:44
  • Both the xml and the itl are pretty small – everything else is caches, artwork files, and the media files proper. Lots and lots of files of wildly varying size. If you work heavily with multi-gig movie files you are bound to eventually run into trouble with CCC backups, but the ever changing XML? I don’t think so…
    – kopischke
    Oct 25, 2011 at 21:22

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