FATAL ERROR :System error 5 . WHY?

[Marco Valenzi]

hello acedb-exspert,
We have Enterprise 3000 with Solaris system 2.5.
when we install the new update WS4 in acedb all go well as far as
update.WS4.4-10 but running update.WS4.4-11 at line 69276 (17%) we see
litle
window with message
 FATAL ERROR :System error 5 I/O error 
diskblock.write cannot actually write the disk ,n=-1

What means ? have you any suggestions ?
thanks
Marco Valenzi