Hide data into Slack space :
1 - What is slack Space When you format a partition, you define a block / cluster size (ie: 64b).
Later, you'll copy files on the filesystem. The slack space is the unused size of the blocks you've written on. Let me illustrate : You copy a 110b file on /dev/sda2 which has a block size of 4096b. Then you'll have 4096-110=3986b of unused space on the block. Let'ssay the sectors are 1024b long, then youl'll have the 3 last sectors free of data.
The remaining space of the first sector is called "datahole", which is the free space that you will leave free before writing into the slack space, to let the OS know the file has ended. Sometimes OS will write meta-datas on this space (for instance, MS-Winblows will use a process called alternate data streams (ADS) to let you have small informations on a file (pictures, size...) in writing behind the datahole of teh last block a file is written on).
If you write a 5321b file, it will use a complete block of 4096b and will write 1225b of data in an other one, leaving 2871b of unused space in which the OS will not read.