Suddenly I'm getting problems where I never used to. I'm using Retrospect 4.0 to back up a Performa 6400 running OS 9.1 to both an external Quantum Fireball hard drive and an APC DAT drive. For two years Retrospect backed up to the external hard drive flawlessly, but now it quits. It reports either error 206 or error 105. Often the Recatalog feature reports "Bad StorageSet Header found (0x010aa600at6439)". (Those last four digits vary.) Sometimes when the back up stops, I hear the external hard drive speed up briefly. Then Disk First Aid reports "checking catalog file. Problem: Invalid PEOF, 13555,1. DFA cannot repair." But Disk Warrior does repair whatever problem that is.
About a week or two after this problem with the external hard drive appeared, the APC DAT drive went into a permanent eject mode, in which it sits there and makes the sound of ejecting a cassette, regardless of whether there's a cassette in there or not.
When turned on, both drives appear in the Retrospect list of SCSI devices. The external's icon appears on the desktop, and all other functions of the external HD work normally.
I have had one or two occurrences when the scanner (the third device in the SCSI chain) caused the internal HD to freeze, but not for a couple of days now.