His drive was single-sided double-density, giving 160k storage. The ones I was using were double-sided double-density, providing 360k of storage. I have no idea

His drive was single-sided double-density, giving 160k storage. The ones I was using were double-sided double-density, providing 360k of storage. I have no idea why the single sided drive had a bit less than half the storage of the double sided one though. There were two "hacks" available with single vs double sided drives and disks. 5.25" floppies had a small notch in the side that, if you covered it with a sticker, meant the drive treated it as read-only. By cutting an equivalent notch in the opposite side of a disk to make it appear writable it was possible to flip it over 180° and use both sides of a double-sided disk in a single-sided drive. Conversely, disks that were rated as single-sided (and hence cheaper) could generally still be used successfully in a double-sided drive, though it wasn't recommended to store anything too important that way because the failure/problem rate was noticeably higher. My guess is that the single sided disks were binned as such due to them failing tests on one of the sides during manufacturing.