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Two identical images backed up, two different file sizes

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#1 trikcard


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Posted 13 August 2013 - 09:01 PM

Okay, this has happened for a while and it's always made me nervous.  Before I do my latest backup, I just want to double check what's going on. 


I have an external hard drive where I keep all my photos, business, etc...  Then I have another external drive that I sync it too occasionally just to back everything up.

However, with the images, sometimes something off happens.  Although I have not touched the images at all, two identical images have different file sizes.  Keep in mind, I have not touched either image and manipulated it at all. 


The main externail jpeg file size is 1,498,512 bytes


The backup jpeg file size is 2,893,394 bytes


Before I just allow my HD to overwrite my backup copy, I just wanted to know the reason for the difference.  Could it be that as I am using the external hard drive, it compresses?  Do both the files contain the same information and I shouldn't worry about it? 

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#2 smax013


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Posted 13 August 2013 - 09:41 PM

It is a function of the "block size" (also called "cluster" or "sector") used to format the drives.

The following page gives some insight:


The gist of it is that when you format the drive, you are basically turning that drive into a bunch of fixed sized "boxes" (aka blocks, clusters, or sectors). If you have a file that is just a hair bigger than the size of one of those "boxes", then you actually need two "boxes" to store the file. If you then copy the file to another drive, that drive might formatted such that it uses different size "boxes". So, on this drive, the file will take up a different number of "boxes" since they are of different size. What the OS then is reporting to you is the number of "boxes" the file takes up times the size of the "boxes", whether all the "boxes" are full or not. So, if the "boxes" for drive 1 are 4 kb in size and the file takes up 2 "boxes", then the file size would reported as 8 kb even if the file actually only takes up 5 kb. If on a second drive the "boxes" are 2 kb in size, then the file would take up 3 "boxes" and the OS would report the file size as 6 kb.

The size of the "boxes" can be a function of your formatting options as well as the overall disk size.


It could also be a function of compression...depending on how you are backing up. If you are just copying the files in the Window Explorer, then there is likely no compression going on. If you are using some sort of backup software, then it is possible that compression is being used...but usually if a backup program is compressing things, it is putting the backup in some proprietary backup file format (i.e. all your files being backed up will be in one big backup file).

Edited by smax013, 13 August 2013 - 09:44 PM.

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