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, especially when a SCSI interface is present and when no device is connected and recognized on the SCSI chain. (For example, you may have only a Zip Drive connected to your SCSI interface; but, it is not powered ON or no Zip disk is inserted.) Evidently, something (e.g. a register or softswitch) involved in the usual GS power-up routine relating to on-line devices gets messed up. Arranging to have some active, on-line device on the SCSI chain (or removing the SCSI card) seems to help reduce frequency of the problem. A nearly certain fix is inserting a 3.5" diskette into Drive 1 before or just after power-up. This usually forces recognition of 3.5" diskette drives and enables correct functioning. ----------------------------