I tried to backup from my NTFS external hard drive to a new exFAT external hard drive. When the copy finished I saw that I have very little free space remained. I checked my file size and I saw that the real size of my file is "30.4GB" but the size on disk is "396 GB".

I must mention that number of file is so high (about 96110 Files, 10807 Folders).

Why has this happened? How should I fix this? I formatted my new hard drive with exFAT filesystem with Allocation unit size of 4096 kilobytes.

  • What about hidden or system files? How many files are there? Which sizes?
    – week
    Dec 2, 2012 at 23:45
  • there are no hidden files or system files
    – Am1rr3zA
    Dec 3, 2012 at 0:11
  • 1
    @week I edit my question , no about 96k files
    – Am1rr3zA
    Dec 3, 2012 at 0:21
  • 5
    Why would you set the allocation unit size so ridiculously high? Dec 3, 2012 at 1:37
  • 1
    The allocation unit is the smallest size a non-empty file can have. If you have a lot of very small files they will each take 4 MB. Consider another filesystem or zipping the files. Jan 22, 2014 at 13:36

1 Answer 1


If my calculations are right than average file size on your exFAT is 331,7KB, but you've set minimal unit size to 4096 KB, which means that there is 92% unused space in every unit, so 96110 (files) x 3768,32 (freespace) = 362173235,2 KB = 345,40 GB (free) + 30,4 (real) = 375,8 GB, still missing few gigs somewhere, maybe because of that average size.

In other words, set unit size as small as possible or use ".tar" or something like that.

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