question about size--on disk

I greatly appreciate your detailed and informative explanation, I now understand better the mechanics of data storage on disks.

Thanks,

Indrek said: ↑

1. The Size On Disk is different from the actual size because drives are divided into clusters, which cannot be shared between two or more files. This means the amount of disk space each file will take up will be equal to or greater than the cluster size, but never less. If the file size isn't a multiple of the cluster size, the remainder of the last cluster is effectively wasted, because you cannot use it to store parts of other files.

The default cluster size for NTFS is, I believe, 4 kB. You can easily check for this by looking at the properties of a small text file.

Now, the reason the difference between actual size and SOD is more than 4 kB for your video files is fragmentation. What I said above about clusters not being shared between files also applies to each fragment of a file, and the difference between SOD and actual size is the sum of the unused space in the last cluster of each fragment.

2. As for why the differences between SOD and actual size don't match up on the two drives, there are two possibilities - either the cluster sizes are different, or (more likely) the file is simply less fragmented on the internal hard drive.

3. The actual sizes are the numbers you'll want to compare, so in this case there's no reason for concern. If you want to be extra sure, you can try creating MD5 hashes of both files and comparing those.
MD5sums for Windows

Hope this helps.

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