Can anyone tell me what is different about MHT and earlier models
wrt to ROM reading ??

MHT have a problem which I think they go into "safe mode" and
only some mods are read 4,6,3D,3E,3F and a few others.
I assume this has something to do with ROM mismatch because if
you mismatch another pcb you will get same result with a good working drive !

Based on above I assume one cannot take ROM v8431 from update and
write this ??

MHT drives have 2 ROM chips - one big Flash ROM (1Mbit maybe) and one small 8-pin chip. The latter is individual for each drive and contains information about the serial number, addaptive parameters, etc. If one swaps pcb of two drives he has to exchange the small chips...

After a little searching I found out :

MHT is based on Marvel core which incorporates the read channel (RDC)
which MHS does not.

The "guys" are still working on Marvell core to enable reading of parallel flash ROM.

BGman - did you try exchanging those "two little chips" and how sure are you that they are serial flash which I think you suggesting ?

Yes I did . With its own serial flash the drive initialize correctly and report the serial number written on the label. With a small chip from a different drive (but same model) it reports the ser. number of the donor drive and has difficulties with reading and writing. The paralel flash is responsible for the firmware of the drive. I repaired a drive with dead pcb by using a pcb from another drive and replacing both the serial and paralel flashes.

useful information. Thanks for sharing guys. If I come across this problem, I'll refer back to this topic.