Amiga high density disk
From Just Solve the File Format Problem
The Amiga high density disk format (3 1/2", double sided, high density) is identical to the Amiga double density disk format, except that each track now has 22 sectors instead of 11.
No Amiga model ever came with a high-density drive. However, the Amiga did support them, and high-density drives were available from third party vendors.
