Disk cloning - How much data will it actually copy?

Question

I am cloning a disk of size 500GB. Disk is not full of data. It has roughly 300GB data. How much actual data it will copy (how much read, and how much write specifically)? What if cloning utility reads a sector, and there’s no data, will it still copy that sector? Or it will skip or command the target disk to write just to next?

I have additional 50GB of such free space on the old disk that has been deleted, recovery tools recovers this kind of deleted data. Will cloning process (software in general) consider this space as data or consider as empty space.

Answer 1

Most data copying tools will only copy the active and undeleted data you have on that storage device.

Data copying tools can fall into three broad categories: File copies, sector copies and partition copies and here’s how it breaks down. If you care about only copying files, then use a tool that allows you to do a pure file copy.

• File Copying: If you have a 500GB hard disk and you only have 300GB of data, the vast majority of data copying tools that do file copying will only copy the 300GB of data. And if you have deleted data on the disk, it will not be copied over; data copying tools only copy the active data on a disk not the remnant data of things that are deleted or removed.

• Partition Copying: But—based on the description and concerns in your question—you might be thinking about full-disk imaging tools that would make a 100% exact copy of the partition on a system. This is the kind of copying that happens with those standalone hard drive duplicator things that look like toasters. Those devices do full partition/disk copying and can lead to issues if you copy a 500GB hard disk to a 1TB hard drive; the 500GB hard disk will be imaged into the 1TB drive leaving 500GB of dead space that can only be recovered via partitioning tools.

• Sector Copying: Some the most basic tools out there—such as dd—are essentially sector copiers and they run a risk of copying deleted data. For example, if you use dd to copy the data with a simple dd if=/dev/sda1 of=/dev/sdb1 then yes, it will clone all 500GB including empty space much like a full partition copy.

Just be sure to use a data copying tool that will strictly copy data and not do a partition clone. For example, Carbon Copy Cloner on macOS is a pure data copying device and I recently used it to migrate data on a 256GB SSD to a 512GB SSD; only the data used on the 256GB SSD was copied over to the 512GB SSD.