When you delete files on storage media like hard drives or USB sticks, as you probably know, the data is not immediately deleted. Instead, the data is just unassigned and will be overwritten by new data as needed. Until this occurs, anyone can restore the deleted data.
To prevent such access to deleted data, on Windows you can use the built-in tool cipher.exe to explicitly overwrite all unassigned storage space. When you do this, you immediately recognize why this is not done by default: it takes a long time to store data to media, even if the data is all zeroes.
This command overwrites unassigned storage on drive C:\ three times, first with “0”, then with “1”, and finally with random values. You better invest a night for this task to complete:
Here is the official description of the “/w” switch used in our example: “Removes data from available unused disk space on the entire volume. If this option is chosen, all other options are ignored. The directory specified can be anywhere in a local volume. If it is a mount point or points to a directory in another volume, the data on that volume will be removed.”