From a MacWEEK article: Iomega's ZipPlus drive should only be used with the AutoDetect cable that ships with it. Using other cables can lead to data corruption. For example, this "precludes the drive's use with PowerBooks, which have a special SCSI adapter cable." Even worse, Iomega said that the drive must be the sole external device on any SCSI chain.
Update: John Garde notes: "You can buy an adapter with a HDI 30 on one end and a SCSI 30 pin on the other. I have one and it works great on my PowerBook and Zip Plus." Larry Sanders adds: "I've used the AutoDetect cable plus a DB25 to HDI SCSI adapter with no problems for the last six months." Still, Iomega's web page on this matter does not recommend doing this (thanks, Rock Maelik).