Without the proper drivers in your System folder, these SCSI devices won't work. The reason is often due to improper SCSI termination. One setup may require a passthrough terminator, while another ostensibly identical hookup will refuse to work unless the terminator is removed. It can be entirely idiosyncratic, or at least apparently so. Such bouncing reflections can make your Mac go weird.
Terminators are devices that stop the bouncing signals.
In practise, my Microtek scanner works just fine with or without any terminator installed. As I said, suggestions, not rules, apply. Most SCSI devices come with either an internal or an external terminator. A few upscale SCSI devices have active termination, which is very convenient circuitry that turns termination on or off automatically as needed. Take anything either say with a grain of salt.