In the early 1980s when I first started working full time, I was in operations. I occasionally reorganized disk packs, which made me very aware that an IBM model 3330-1 drive held 100 MB of data and its double density cousin the 3330-11 held 200 MB.
Much later (in the Wikipedia era), I learned Digital Equipment Corporation had used 3330-1/3330-11 compatible drives on their systems as the RP04/RP06 drives.
This weekend I went to install an emulated PDP-11/70 image, and discovered that while the legacy IBM and DEC systems may have used similar drives (in fact, the media on the real drives is interchangeable), how they use them isn't the same. If you write full tracks (13030 bytes, a number I can rattle off from memory 40 years later) as IBM users do, you can fit the aforementioned 100/200 MB per volume. But DEC environments write 512 byte sectors (which imposes more overhead per track), and it reduces the 200 MB capacity to 176 MB.
This confused me no end when I was looking at the RP06 webpage and the "same" drive had different numbers than what I knew.
"It's an IBM model 3330–11 drive, but not as we know it, Captain."
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