That is a SCSI hard drive. The thing that gives it away is the Apple "40" sticker. This means it is 40MB capacity. The first Macs to use an IDE drive were the Quadra / Performa 630s and they had 250MB drives. Any 40MB Apple-branded drive will be SCSI.

The problem you're experiencing may be SCSI related. Try adjusting the number wheel on the back of the hard drive unit so that it reads "6". When the Plus scans for SCSI devices it starts at 7 and counts down, so it'll find a drive with ID 6 quickly. Also try booting it without the SCSI terminator.