>   To make a long story short, I did a little digging, and a
> databook for the ST506 interface says the sector size can be
> 128, 256, 512, or 1024 bytes but no higher :(, so I think all
> modern drives should support 1024.  The savings look like at
> least 39 bytes saved per removed header&gap.

The ST506 spec is irrelevant to SCSI and ATAPI disks.  I've seen SCSI disks
which supported sectors as high as 2048 bytes, but I bet no current drive
made supports anything other than 512 bytes, period.  (I *know* that none of
my current drives can be formatted with any other size.)

It turns out that a certain very popular virus loader named MS-DOS does not
support sector sizes other than 512 bytes on hard disks, so the ability to
reformat disks with other sizes has gradually disappeared.  (I have taken
advantage of it on Mac computers, hence my disappointment that modern drives
don't handle it.)  Furthermore, with modern encodings, there is (in fact)
less ability to play with the sector sizes:  Zone Bit Recording drives (i.e.
essentially every drive manufactured now) have different numbers of sectors
per track in different parts of the drive, so making the sectors larger might
actually end up wasting space (for transistion cylinders where there isn't
enough room for one more 1024 byte sector, even though one more 512 byte
sector can be added).

Also note that IBM has a line of drives which have one physical sector per
track!  The logical sectors are implemented purely in software, so if you
write only to one sector of a given track, the drive still has to read the
whole track and write the whole track back out.  (Fortunately, this is no
slower than what the drive would have done anyway.)

(However, it wouldn't surprise me at all to eventually see drives with 4K
sectors and "optimized for NT 5.0" stickers...)