[manyturtles]

IDE was just coming in (in the UK) in 1990. The acronym got updated to "AT Attachment" because "Integrated Drive Electronics" was generic, and it wasn't as if the older drives had no electronics on them. Much later when SATA showed up, the name evolved again as ATA became known as Parallel ATA to distinguish the two.

Before that, when you installed a hard disk you had to go into the BIOS to specify the geometry of the drive. 46 types were already defined, to match individual drives on the market. "Type 47" allowed -- required -- manually specifying the drive geometry in terms of cylinders, heads and sectors. So for a short while some traditional MFM or RLL drives would be informally classed as Type 47 because their geometry and capacity differed from earlier drives.