Check that the correct drivers for the SCSI device are loaded in the Device Manager.
External hard drives and most removable cartridges must be "mounted" so that they appear on your desktop like a floppy disc or hard drive. The device driver generally does this automatically. Some devices require a little bit of help for the computer to recognize and mount the device. A SCSI utility program like SCSI Probe or FWB Hard Disk Toolkit will be handy if your computer is having difficulty mounting a SCSI device.
In removable cartridge drives like Zip drives, there is a possibility that you might have a bad cartridge. Try a known good cartridge or test the problem cartridge on another drive mounted on another computer.
Check for:
Incorrect termination of the ends of a SCSI daisy chain.
Duplicate device ID numbers in a daisy chain.
Drive not powered on.
SCSI interface not enabled.
SCSI interface has resource conflicts with another device.
Incorrect cable used to connect the drive.
