Repeated disk crashes with ISC 2.2 & ESDI drive

[Geraldo Veiga]

I posting this in the hope that someone could give me a hint of what is
wrong with my hard disk.

Every once in a while after I shutdown UNIX (with powerdown), and
before I actually turn the power off, the boot sector of my hard disk
gets wiped out.

This has happened 6-7 times ever since I installed this HD (6 months).
In the first few times it was a disaster, as I had to reinstall the
OS from scratch.   More recently, I've been keeping a backup of the
boot sector (thanks to pboot), and restoring it brings back the whole
disk.

This problem has never happened by simply turning the machine off
(neither under DOS nor under UNIX).  It seems to be connected with the
shutdown procedure.

Setup:  MR BIOS, OPTI chipset motherboard, Adaptec 2322D controller,
	Maxtor 4380 HD, ISC 2.2.   I have two 32M partitions for DOS,
	and UNIX takes up the rest.

Questions: What could be going on?  Is it just a flaky disk acting-up
	   on occasion?

Thanks for any pointers.