Ruud,

I'd look at the Fibre Channel Adapter on the ESS.  It could be functioning
just well enough to allow a stable link to your SAN (therefore your paths
will look good), but it may not be seeing the DASD inside the ESS.   FCAs
are hard to troubleshoot because a loop back test isn't a very reliable
test.  All it tells you is that light is traveling successfully around the
loop - it doesn't prove that actual communications are occurring.
Good luck on this one, and please let me know what you find out!
Thank you,
Jeff Caffey
Enterprise Systems Programmer
(AIX & Storage Administrator)
Pier 1 imports, Inc.  -  Information Services
jlcaffey AT pier1 DOT com
Voice: (817) 252-6222
Fax:   (817) 252-7299