Hello.

To provide some updates on my earlier questions (for future generations
googling through the archives).

So, my question is, does anyone have an idea on what could cause this, and
what would be the best way to diagnose/fix the lockup problem?  Thanks in
advance.

Since then I have re-created the original 3-device RAID5, it synced without
lockups. Perhaps I should write them off as some hardware bug, especially
considering that the nForce chipsets aren't exactly known as the gold standard
of bug-free design.

So why the same RAID5 to RAID6 conversion started so differently in these two
cases? And is it even possible to reshape RAID5 to RAID6 while simultaneously
adding a disk, without overwriting all the other disks' contents (it surely
looked like this is what was happening in the first case)?

Yes it is possible; I should have read about the --layout=preserve option in
the mdadm man page.

-- 
With respect,
Roman