Boot from a floppy and run SCSI probe to have a look at the bus and see if it's advertising itself.

You have two problems...

1. As I understand it the Mac Plus requires drives that default to using a 512 byte sector ( or some similar initialisation ), later Macs did a negotiation with the drive to set this up. I use an old 40Mb drive for this very reason.

2. The original SCSI hard disk setup program only recognizes Apple supplied drives, unless you either using the A/UX version or patch it.