This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
This distinction is at the eletrical signalling level. However, single-ended is limited to total bus lengths of 6.0 meters, while differential can go up to 25 meters (SCSI-II). Differential is generally more robust to noise and cross-talk, but the bus drivers are more expensive. In theory no difference in transfer speed or capabilities, but in practice the added noise margin could mean higher reliable transfer rates on your system, especially if your bus is long. Most disk drives and most low-end products are available only with a single-ended interface. A few devices are available with either as a purchase option, and a few are switchable by the user. The cables and connectors are the same for both, though the pinouts are (naturally) somewhat different. Plugging a single-ended device into a running differential bus or vice-versa may result in damage to one or more devices. Most newer devices have fuses or protection circuits utilitizing the DIFFSENSE signal to prevent device damage. There are now recommended icons used to distinguish between the two: single-ended differential /\ //\ / \ // \ < -- << -- \ / \\ / \/ \\/ Converters do exist that will allow you to hook up single-ended devices to a differential bus and vice-versa. People who have used them say they work great, but in theory they shouldn't work :-). As I understand it, changing the signalling introduces delays in some of the control signals that means that some devices could miss certain signal transitions. The best advice is to borrow one and try it, and see if it works in your system. One company's name is Paralan, (619)560-7266.