This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
This problem seems to crop up from time to time when a SCSI interface is present, especially when no device is connected and recognized on the SCSI chain. Evidently, something (e.g. a register or softswitch) in the usual GS power-up routine relating to on-line devices gets messed up. Inserting a 3.5" diskette into Drive 1 before or just after power-up usually forces recognition of 3.5" diskette drives and enables correct functioning. By: LJ Silicon
 
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