When I drag and drop 1 external HD to another for back up the sizes dont match

[davidnssbm]

This is very odd to me. I have two external back ups. when I drag and drop 1 to the other to update the back up, the sizes of the HD dont match when I check GET INFO. This even happens when I drag and drop ALL of the files. For instance.

I have two brand new 1TB HDs which I use for a project I'm editing in FCP. When I drag and drop all 133GB from HD1 to HD2, and an hour later upon completion it says HD2 has 125GB, but I got no errors...

Also, all the files seem to be there! When I check the SIZE in the LIST view in the finder, all my folders match. Yet at the bottom my total available is also different.

what gives?

 

[dashcs]

Here the thing.
If you get info on all the folders(hold option + command then press i).You get the correct size however if you get info on the entire drive,you get something else that doesn't add up to the folders.

On your drives,you have hidden files/folder.
Most common one is spotlight and trash that could be related to your issue

So if you have something in your trash icon on your computer that was from the external HD.Its still on the external HD but its inside the hidden trash folder that your trash icon on your dock is pointing to.

Also your computer does index your external HD inside the hidden spotlight folder on the external HD.

 

[name99]

dashcs is correct.
(a) There are various hidden files on your drive that will not be copied over via the drag copy you describe. These include the files used for the HFS+ journal, spotlight indexes, the fsevents log (used by Time Machine) and so on.
Even apart from those, the file system data structures like the catalog tree can be slightly different sizes because over time these develop holes which are not filled in immediately; but when you do a mass copy to a different drive, the new trees are created from scratch without holes.

(b) If this bugs you, the correct way to do this sort of copying is via the "Restore" tab in Disk Utility.
Open Disk Utility, choose "Restore", and then drag the source drive to the source field, and the dest drive to the dest field. If the two drives are the same size, or at least similarly sized, this will make a byte for byte accurate copy of the drives, if that is what you want. If the drives are full (say 80% full or more) it is also the fastest way of copying a drive; but not if the drive is fairly empty.