That was the case until recently. On new Macintosh computers, the checksum verification works. Some SCSI drivers improperly implement the new partitioning scheme. If this checksum fails, the driver is not loaded. This checksum algorithm is documented in Inside Macintosh , Volume V-580, The SCSI Manager.
Devices fails to include the disk driver checksum algorithm.
The checksum routine tests the number of bytes specified in pmBootSize , beginning at the start of the driver boot code. Only drivers contained within the new partition map have this test performed. If you are using the old partition map scheme documented in Inside Macintosh , Volume IV-283, The SCSI Manager, the driver does not have its checksum validated. SysEnvirons was created for compatibility reasons. It allows an application to make a single call to the system to determine its characteristics.