hard_drive_0_device
The option documentation is properly formatted yet
Example: HD0
By default, for mounted folders, the device name is DH0 for hard_drive_0, DH1 for hard_drive_1, etc. You can override this on a per-drive-basis.
Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/hard_drive_0_device