Hard-sector discs -- how many sectors?

FAIKL there's no such thing as SS 3" _media_ -- all the disks are coated (and tested) om both sides. There is 40 cylinder and 80 cylinder media. and Amstrad (the main user of 3" drives) confused the situation by using single-head 40 cylinder drives and double-head 80 cylinder drives only. So the 80 cylinder disks got known as the double-sided ones. However, 40 cylinder double-head drives exist, I've used them. The plastic shell of the 3" disk has an offset notch in the front edge. A double-head drive has a metal bracket that fits into this notch, so you can't put the disk in 'flipped'. A single-head drive either ingores this notch or has a light barrier to detect it -- the single-head drive in my Tatung Einstein has am 'in use' LED that is red if the disk is in one way up and green if it's in the other way up. This means, of course, that a double-head drive can't read the flip side of a single-head disk. You can't flip the disk to read it with head 0 (because of the bracket) and you can't read it with head 1 because the disk is effectively turning the wrong way. AFAIK this is the only type of disk that has this problem (Yes, 8" dishs have the index hole in a different place on single and doubled sided media, but every 8" double-head drive I've seen has 2 index sensrs so as to be able to handle single-sided media too) -tony