This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Almost immediately after power-up, your SCSI interface begins checking for devices. Most likely, the problem is that your SCSI interface checks the Zip Drive before it is ready and decides no disk is present. The cure is to put your Zip Drive, CD-ROM drive, etc. on a separate power line. These devices should be switched ON 5-10 seconds before powering- ON the computer. If you depend upon reading SCSI setup information from your Zip Drive-- like, if it is the only write-able device on the SCSI chain-- then a Zip disk on which the setup info has been saved should be inserted after the drive is switched ON and before powering-ON the computer. ----------------------------
 
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