With plain vanilla 2.6.11-rc4 the same bug appears after about 250GB 
(avg of 2 trials). With the TBG clock setting line omitted it still 
happens, but after about 1 1 TB (avg of 2 trials, takes about 6hrs per 
trial). Interestingly enough, this change does not slow down the setup, 
it even seems a little faster.

I was mistaken earlier: the 4 drives are not exactly the same, there is 
2 6B200M0 one 6B200S0 and one 6Y200M0. This should be irrelevant as I 
have swapped disks and wires and the problem happens anyway. One 
interesting thing: in init 1 the timeout seems to appear faster, after 
about 200GB in the case with the omission. I would be inclined to think 
this is some sort of a deadlock or race condition: the kernel does not 
dump or panic, it just hangs on pdc_eng_timeout. When we dumped the 
stack  in that function, all we had was pdc_eng_timeout, as there seems 
to a be a separate thread per disk that gets waken up for error handling.

Any ideas on how we can catch this one?
TIA,
Francois