Format Disk

1. Press the Master/Global button.
2. Enter 77 on the numeric keypad (or use slider to find 'FORMAT DISK').
3. Use the data slider to select your hard disk drive.
4. Press ENTER.
5. When prompted 'Are You Sure?', press the Inc/Yes button to confirm.

Source: Emulator IIIx — eiiix-operations-manual (manual)