We look at the number of sectors per track for these old drives, and give examples from the seagate archive.
Almost all MFM drives have 17 sectors per track, like the prototypical ST506, with 512 bytes per sector. The st419 had 32 sectors per track (geometry C/H/S=306/6/32, total capacity 15.0 MB). Maybe also 26 occurs.
Almost all RLL drives have 26 sectors per track with 512 bytes per sector. (And these may well be the same drives that have 17 sectors per track with MFM; indeed, RLL is a denser recording technique.) The st225r had 31 sectors per track (geometry C/H/S=667/2/31, total capacity 21.2 MB), just like the st250r (with 4 heads, twice as large). Also 25 occurs.
ESDI drives can be soft sectored or hard sectored dependent upon the controller used. Hard sectored controllers use 34 sectors per track and soft sectored controllers can use 34, 35 or 36 sectors per track. The drive must be jumpered for hard/soft sectoring to match the constraints of the controller. (Here we see the first traces of the "translation" disease: For systems with BIOS types that only support 17 sectors per track, the controller may be jumpered for translation mode. In ESDI translation mode, the controller converts the drive to a 17 sectors per track drive by logically doubling the number of physical heads.) A few other values occur: 53 (e.g. the Seagate st4766e or the Maxtor 9780E)), 54 (e.g. the st2182e), 63 (e.g. the st4767e), 71 (e.g. the Maxtor XT 8800E). Also 26 and 72 occur.
IDE (ATA) drives usually have a variable number of sectors per track and here we see that one has to use a geometry that is unrelated to the actual hardware. Sometimes only a single geometry (specified by the manufacturer) will work; usually any geometry that multiplies to the right total capacity will be OK.
Thus, we see a great variety of values for the number of sectors per track here, and it is not always clear in which cases this is an obligatory value. Examples: 16 (e.g. the st9100ag), 17 (e.g. the st325ax), 24 (e.g. the st9190ag), 27 (e.g. the st280a), 28 (e.g. the Maxtor 8051A), 29 (e.g. the st1162a), 32 (e.g. the st9051a), 34 (e.g. the st3195a), 35 (e.g. the st3283a), 36 (e.g. the st1239a), 38 (e.g. the st3211a), 47 (e.g. the st9150ag), 50 (e.g. the st3291a), 51 (e.g. the st9385ag), 52 (e.g. the st9240ag), 53 (e.g. the st3271a), 55 (e.g. the st2274a), 56 (e.g. the st2383a), 59 (e.g. the st9550ag), 60 (e.g. the st9300ag), 61 (e.g. the st1401a), 62 (e.g. the st3385a), 63 (e.g. the st3270a).
Of course, all modern drives use 63 sectors per track.