This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
The UniDisk 5.25 was Apple's replacement for the venerable Disk ][. It featured an updated design, tan color plastic case, a DB-19 cable, and daisy- chain port on the back to which other drives could be connected. The case color was later changed to platinum and the drive was rechristened to the "Apple 5.25 Drive". The UniDisk 3.5 was Apple's first 3.5" drive for the Apple II line. Like the Unidisk 5.25, it had the DB-19 cable and daisy chain port on back. UniDisk 3.5 was dropped in favor of the Apple Disk 3.5, a different design, at the time of the launch of the IIgs. ---------------------------------- By: Mitchell Spector One important difference you forgot to mention: The UniDisk 3.5 was an "intelligent" drive, whereas the Apple 3.5 was a "dumb" drive. There was a specialized circuit board inside the UniDisk 3.5 drive which had its own processor, memory, IWM controller and firmware. This was needed for the //c, in order for the drive to process data before sending it on to the machine (which was too slow to do the job by itself). You can easily spot a UniDisk 3.5 as it matched the snow-white color of the //c and had i ts access LED and eject button above the disk insert slot (the Apple 3.5 has them in-line). By: Rubywand