Apple does not support the connection of SCSI hard disk drives and other SCSI devices such as scanners, Zip drives, or JAZ drives to iMac and iBook computers. Apple does not intend to develop an adapter or cable that would translate SCSI signals to USB or vice-versa, although a number of third-party companies have developed such an adapter.
Related Articles on Connecting Older Peripherals to iMac computers:
For further information on USB in Apple products, visit Apple's USB site:
http://www.apple.com/usb/
To find USB products for your Macintosh, visit the USB area of the Macintosh Products Guide: http://guide.apple.com/uscategories/usb.html