A 1TB disk is designed to store 1TB. This means that its physical storage can hold 1TB.
On the logical level (OS, file-system) there are some mechanisms which may obfustace this fact:
- Compression: If the OS support file-compression, the physical storage size needed to store files can be smaller than the actual file size.
- Mark-as-deleted: Usually when a file is being deleted, the region on the disk where the file is stored is not cleared. Instead, it is only marked as free, and as long as this region was not reused - the file may be recovered - just by indication that there is a file stored in this region.
Once a physical region of the disk is rewritten, it is impossible to restore the previous data using software. However, in mechanical disks, head positioning system is not exact enough. New data written to a drive may not be written back to the precise location of the original data. This track misalignment is argued to make possible the process of identifying traces of data from earlier magnetic patterns alongside the current track.
Electron microscope based hardware were built to probabilistically restore data based on this fact. Note that it would require to dismantle the drive in a clean-room environment.
The following study shows that for modern drives, although there is a good chance of recovery for any individual bit from a drive, the chances of recovery of any amount of data from a drive using an electron microscope are negligible.