Thanks for the update and confirmation that it is a floppy port drive. It is a later drive than the one I know about, but very closely related.
Try: plug the serial connector into the Mac's printer port, plug floppy connector to floppy port, add power to disk drive whilst waiting twenty seconds, then power on Mac Plus. If that fails, repeat using modem serial port. If both fail, accept the fact that the drive is not bootable on a Plus. Adding an INIT to the System Folder isn't going to make a difference. An INIT only executes when the system unit boots from a System Folder on a drive, and your system unit isn't doing that. You don't need the HD20 INIT -- a Mac Plus already knows how to cope with HFS drives from its ROM code (128KB ROMs).
Nudge: model number and description on the case...
Try: plug the serial connector into the Mac's printer port, plug floppy connector to floppy port, add power to disk drive whilst waiting twenty seconds, then power on Mac Plus. If that fails, repeat using modem serial port. If both fail, accept the fact that the drive is not bootable on a Plus. Adding an INIT to the System Folder isn't going to make a difference. An INIT only executes when the system unit boots from a System Folder on a drive, and your system unit isn't doing that. You don't need the HD20 INIT -- a Mac Plus already knows how to cope with HFS drives from its ROM code (128KB ROMs).
Nudge: model number and description on the case...