I received instantaneous replies from:
Oisin McGuinness 
Bill Bennett
Andres Henckens
The first two suggested that I run uerf with the "-o full" option
which indicated:
Hard Error Detected
DEVICE NAME                             Quantum XP34300W
MEDIUM ERROR - Nonrecoverable medium_error
Medium Error bad block number: _5082849
etc. so that I conclude that the disk needs to be replaced.
For the record (as correctly requested by Bill):
The machine is an AlphaStation 600/266 running Tru64 Unix v4.0
(unpatched) and the sisk is indicated above.
I also ran:
# scu -f /dev/rrz1c
scu> verify media
Verifying 8813920 blocks (0 - 8813919) on /dev/rrz1c (XP34300W), please
be patient...
scu: Verify error at logical block number 4634584 (0x46b7d8).
scu: Sense Key = 0x3 = MEDIUM ERROR - Nonrecoverable medium error,
     Sense Code/Qualifier = (0x11, 0) = Unrecovered read error
scu: Error number 1 occurred on Thu Jul 15 16:47:40 1999
which more or less confirms my fears.
Thanks for very quick advice.
My original question is included below.
Regards,
Peter
Peter Stern
Chemical Physics Department
Weizmann Institute of Science
76100 Rehovot, ISRAEL
email:  Peter.Stern_at_weizmann.ac.il
phone:  972-8-9342096
fax:    972-8-9344123
------------------------------------------------------------------------
We get the following errors from uerf -R and system response is very
slow.  What does it mean?  Is it a disk hardware problem or something
else?:
                                                  uerf version 4.2-011
(122)
********************************* ENTRY     1. *********************************
----- EVENT INFORMATION -----
EVENT CLASS                             ERROR EVENT 
OS EVENT TYPE                  199.     CAM SCSI 
SEQUENCE NUMBER                 24.
OPERATING SYSTEM                        DEC OSF/1 
OCCURRED/LOGGED ON                      Thu Jul 15 15:44:07 1999
OCCURRED ON SYSTEM                      quantum 
SYSTEM ID                 x0005000F
SYSTYPE                   x00000000
----- UNIT INFORMATION -----
CLASS                         x0000     DISK 
SUBSYSTEM                     x0000     DISK 
BUS #                         x0000
                              x0008     LUN x0
                                        TARGET x1
(over 20 such entries)
Received on Thu Jul 15 1999 - 14:11:15 NZST