Videographics Corporation Technical Support sent us an email that explains the situation in detail and offers some work-arounds. We have posted it as a new MacFixIt Extra on SCSI ID Numbers and Multiple Bus Systems.
Matthew Wilkins (of La Cie Technical Support) similarly explained:
Usually when SCSI devices do not agree when they have the same ID numbers on different busses, it indicates that the driver software is not fully SCSI Manager 4.3 Compliant. If this is the case, the user will need to either find out if the manufacturer has SCSI Manager 4.3 compliant drivers, or the user needs to install Apple's SCSI Manager 4.3. Normally, this is included in the System file.
Bradley Dichter noted that this is not a new problem and can occur with any software that is "not fully SCSI Manager 4.3 savvy." Scanners and removable drives appear to be especially "problematic."
Several readers (including Ryan Tanaka, Bob Sain and Chris Halaska) all reported a documented problem specifically with UMAX scanners (including the Astra 1200S and the Astra 610S). Essentially, as confirmed by UMAX technical support, the VistaScan software does not recognize the dual bus setup. It will not work if the scanner has an ID that is also in use by another device on another bus. Bob was told by UMAX technical support that "the scanner 's ID has to be ID #2 and that no other device, either internal or external, could use the same ID."