Why are the exact same files I copied from one external hard drive to another different in size. (Not “size on disk”)

Understanding File Size Discrepancies When Transferring Data Between External Hard Drives

Managing digital storage and ensuring data integrity can often lead to perplexing observations, especially when the apparent size of files changes during the copying process. Recently, a user shared their experience transferring photos from a 1.5TB external drive to a new 24TB Seagate Expansion Drive, raising questions about why identical files can differ in reported size after copying. Here, we explore the common reasons behind such discrepancies and provide insights to help you better understand your storage devices.

Case Overview

The individual transferred a collection of family photographs, amounting to 66.6 GB on the original 1.5TB drive, onto a new 24TB external drive. Interestingly, the total size reported on the source was significantly less than the space occupied on the destination, with the destination showing approximately 168 GB used for the same set of files. Additionally, during the copying process, some files and folders failed to transfer seamlessly, prompting questions about hidden or system files, file naming conventions, and storage allocation.

Why Do Files Change Size After Copying?

There are several reasons why the size of duplicated files and folders can vary between source and destination:

File System and Cluster Allocation
Different storage devices may use varying file systems (e.g., NTFS, exFAT, APFS) and cluster (allocation unit) sizes. Larger cluster sizes mean that small files occupy more space on disk than their actual data size. For example, a 20 KB photo stored on a drive with a 128 KB cluster size will take up 128 KB on disk, inflating apparent storage use.
Hidden Files and System Metadata
Operating systems often store hidden files, system caches, or metadata that are not immediately visible to users. MacOS, for instance, creates hidden .DS_Store files, and other hidden files can also exist. Even if these are not explicitly visible, they can contribute to the total disk usage.
File Names and Formats (Case Sensitivity)
Files with the same name but different case extensions (e.g., .jpg and .JPG) are stored separately, especially on case-sensitive file systems. Transferring these files can lead to duplication, increasing total size.
File Compression and Metadata
Some file formats or storage configurations include compression or embedded metadata, which can affect how files are