We had the same annoying problem for almost a year.  Open call
with Legato finally discovered the problem.  We had isolated this
to a few clients but could not seem to find out what was hanging
the drives.  Clients would stream with a good speed and then
abruptly halt.  Drive would then become useless and all save sets
from other clients writing to that drive would also fail.  We
found the error to be a tcp setting on the HPUX client.  Seems like
the users had a program that used ndd to set the tcp_keepalive_interval
to 60 seconds from the default of 2 hours.  The save
sessions would time out and close the tcp sessions.  Once the parameter
was reset, we have never seen the condition again.  I suggest you
take a look and see if a particular client or set of clients are
in the savegroup that is causing this condition and troubleshoot
from there.  Good luck.

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