Backing up to external drive

[applelost]

Scenario:

I copy a large folder (40 gb) over to an external drive, to free up space on my HD. It shows up as 40gb on my external. I delete the folder to free up space on my HD. Everyone happy.

A couple days later (I know I know it should have been sooner, in case that external failed) I copy the same 40 gb folder from the external to another external, to have a second backup of it. But this time the file shows up as 39.6 gb. Hmmmm.

I have also noticed this happening is the past, sometimes the folder will do the opposite, and show up larger on an external, say 40.3 gb instead.

Is this normal, to have discrepancies in folder size this much, even if all files are copied properly? Or does it mean some of the files within the folder did not copy properly.

 

[FreakinEurekan]

I can think of a couple scenarios where this might occur, without meaning there were errors in the copy.

Do both externals have the same format? (HFS+, exFAT, etc)

Do they both use the same block size?

If format or block size are different, reported total usage for a folder full of files could be rather different.

 

[applelost]

I can think of a couple scenarios where this might occur, without meaning there were errors in the copy.

Do both externals have the same format? (HFS+, exFAT, etc)

Do they both use the same block size?

If format or block size are different, reported total usage for a folder full of files could be rather different.

My knowledge on the tech front is very limited, all I know is that they are both Western Digital My Passport drives specifically for Mac. So I assume they are the same. I think I reformatted one a while back to erase it, not sure is that matters.

 

[applelost]

Does anyone else have any suggestions, given that they are the same brand/type external drive?