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I tried to backup from my NTFS external hard drive to a new exFAT external hard drive when my 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.

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What about hidden or system files? How many files are there? Which sizes? – week Dec 2 '12 at 23:45
there are no hidden files or system files – Am1rr3zA Dec 3 '12 at 0:11
You copy single file 30.4GB? – week Dec 3 '12 at 0:15
@week I edit my question , no about 96k files – Am1rr3zA Dec 3 '12 at 0:21
Why would you set the allocation unit size so ridiculously high? – David Schwartz Dec 3 '12 at 1:37
up vote 5 down vote accepted

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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