Data Streams in NTFS allow files to have hidden content that is harder to see.




You can see how much space is used in Windows Explorer, going to properties for a file/folder. The Size field is the main file/folder. The Size On Disk includes the main file as well as any alternate data streams.





Create a data stream from command prompt: notepad notes.txt:test





To view them in command prompt, use: dir /r




Copy a file into a data stream:

type picture.jpg > notes.txt:pic1



Notes:

Deleting the main file will delete alternate data stream.

Copying to a different file system (such as FAT32) will warn you that some properties can't be copied (the alternate data streams).



Nirsoft utility "AlternateStreamView" lets you scan for them.

https://www.nirsoft.net/utils/alternate_data_streams.html



More info:

https://www.youtube.com/watch?v=3BWTo87oCwc