My childhood friend reached out about wanting to hide her music collection. This completely nerd sniped me. What I did was create a FAT-like FS in complete reverse (starting at the end of the disk). AES-CTR was used to encrypt it (hurrah plausible deniability), so it would merely look like random empty space. The idea was then to drop some decoy files on the drive, defrag+compact, and then use the free space for the music collection.

She decided against crossing the border with the music collection, and so it was never used. Still a fun steganography project.