since i don't have one of these drives, i wrote a little
program to dig into the ident block.

a small aside.  ata disks have two sector sizes.  (groan.)
there's the logical sector size and the physical sector size.
by default they are both 512.  the ears drive sets the
set logical sector size to 512 and the physical sector
size to 4096 by reporting that there are 8 logicals/physical.

here's the output of this program
3,907,029,168 4096-byte sectors
serial: WD-WCAD-WCAZA995782551.0AB51WDC WD20EARS-00MVWB0
firmware: 51.0AB51WDC WD20EARS-00MVWB0
model: WDC WD20EARS-00MVWB0
pflag: llba smart nop atapi16 sct

note that there are not 3e9 4k sectors there are 3e9/8 4k sectors;
fis has got that wrong.  i've never had an 4k sector drive to test
with.

the way to interpret this information is you may use 512
byte sectors if you really want to suffer terrible performance
(usually 1/3 the normal performance for reasonablly random
workloads.)

let me think a bit about the correct solutions to this.  it's clear
to me that we just can't assume 512-byte sectors any more.

in the mean time, you can change this

; diff -c /sys/src/libfis/fis.c /tmp/fis.c
/sys/src/libfis/fis.c:320,326 - /tmp/fis.c:320,326
  		f->lsectsz = sw * 2;
  	if(i & 1<<13)
  		f->physshift = i & 7;
- 	return f->lsectsz * (1<<f->physshift);
+ 	return f->lsectsz /* * (1<<f->physshift) */;
  }

and then recompile your libfis and your kernel, and you
should be good-to-go.

- erik