Why are my copied files smaller than normal?

Hello,

may anyone please tell me why is it that when I attempt to copy a 150GB folder from one location to another, the resultant copied folder will only be about 110GB+ in size with obviously missing contents?

If it makes any difference, I'm copying the folder from an external hard disk to one inside my PC.

Any insight will be most appreciated.

Thank you. :)

Edit: Here's a more thorough description of what's actually going on. When I right click on the Properties of the folder, the size of the folder is about 150GB. But when I copy the folder and attempt to paste it into a separate location, it says 'Copying 117GB' and when the processes finishes, the resultant folder will also be 117GB in size.

When I look into this newly copied folder, there are plenty of very obviously missing subfolders which have not been copied. Subsequent to that, when I tried copying some of these missing subfolders, in turn, these can actually be copied over.

My original folder has far too many contents for me to individually check if each subfolder / file has been successfully copied. In that case, may I ask for the possible causes for this error and how I can ensure a successful copying process?

Thank you.