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360K Floppy in 1.2M Drive


Writing to (which includes formatting) a 360K floppy in a 1.2M drive results in a 360K floppy that does not conform to the 360K standard.  See note 1 below.
If that floppy is then placed in a 360K drive, the drive may or may not be able to read the floppy, either fully or in part.

In the IBM DOS 3.3 reference, IBM includes on the subject, "IMPORTANT: If you write on any of these diskette types using a high-capacity drive, you may not be able to read the diskettes in a single-sided or double-sided drive."  See note 2 below.

Some technical information on the subject is at here.

Some more information on the subject is discussed on pages 538 and 556 (actual pages, not PDF pages)(and some other pages) of the book at here.

These issues are why IBM offered a 360K drive as an optional extra for the IBM 5170 (IBM AT).  IBM 5170 customers who needed to create/write 360K floppies for others that only had a 360K drive, would use the 5170's 360K drive (and double density media) rather than using the 1.2M drive.  As a result, 'true' 360K floppies were created, and the recipient's 360K drive guaranteed of being able to read the floppies.

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Note 1	A 360K drive is accurately, a 48 TPI, double density (DD), 5.25", floppy drive. A 1.2M drive is accurately, a 96 TPI, high density (HD), 5.25", floppy drive. By '360K standard' I mean: - the track width used in a 48 TPI double density floppy drive; and - written using a magnetic coercivity of 300 Oe (290 according to here).
Note 2	IBM's "double-sided" drive is what is later became commonly known as a 360K drive. IBM's "high-capacity" drive is what is later became commonly known as a 1.2M drive.