This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
For hard drives, "SCSI-2" basically means that the drive supports a stricter command set. The physical interface is usually identical. For other device types, "SCSI-2" means a lot more, because the original SCSI standard didn't define much in the way of device types and command sets, so most devices use proprietary command sets. SCSI-2 standardises the command sets for most types of devices. There are three special types of interface that you might see mentioned: "Fast SCSI" supports data transfer at twice the speed of the original SCSI standard (10 MB per second vs 5 MB per second). This will not be a compatibility issue, as it is just the maximum transfer speed supported by the drive. The Apple II cannot transfer more than one megabyte per second. "Wide SCSI" uses a different cable arrangement to double the width of the data path (16 bits instead of 8 bits). A wide SCSI drive cannot be used with an Apple II, unless it can also operated in "narrow" mode with the original 50-pin connector. (There is also "Fast Wide SCSI", which doubles the data rate and the width of the bus.) "Differential SCSI" involves a different type of interface to the computer, where every data signal has a balanced positive and negative pair of wires, rather than a single wire and a ground line. I believe it has a different type of connector. Differential SCSI drives cannot be used with an Apple II." Some drives use a proprietary connector, but the standard (narrow, non- differential) SCSI bus uses the same 50-pin connector for SCSI-1 and SCSI-2. The only significant problem you might run into is termination, and supply of termination power. SCSI-2 devices tend to be fussier about termination than older devices. ------------------------------