I agree with Steve, above, about pending (or existing) disk failures. You might want to look into the smartmontools suite as well. You can schedule tests for your disks, and query them directly about their status.

write back with any info from smartctl when you've got it.

to run a bunch of short tests on all your disks, just do:

for disk in /dev/hd? ; do
  smartctl -t short "$disk"
done

There should be some messages printed about what time you should expect the tests to complete. once the tests have completed (probably 5 minutes max for most short tests, but depends on the disks), you can read the info with:

for disk in /dev/hd? ; do
  echo "===${disk}==="
  smartctl -a "$disk"
done > diskreports.txt

You should then be able to read the diskreports.txt file to see what the disks have to say for themselves.

For a single disk, of course, the commands are even simpler:

smartctl -t short /dev/hdX
## wait until the suggested time
smartctl -a /dev/hdX

documented on: 29 Sep 2007, dkg

% smartctl -t short /dev/sda; sleep 2m; printf '\a'
smartctl version 5.37 [i686-pc-linux-gnu] Copyright (C) 2002-6 Bruce Allen
Home page is http://smartmontools.sourceforge.net/

Sending command: "Execute SMART Short self-test routine immediately in off-line mode".
Drive command "Execute SMART Short self-test routine immediately in off-line mode" successful.
Testing has begun.
Please wait 2 minutes for test to complete.
Test will complete after Sat Mar 29 15:46:13 2008

Use smartctl -X to abort test.

% smartctl -a /dev/sda

Short self-test routine
recommended polling time:        (   2) minutes.

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed without error       00%     27420         -

documented on: 2008-03-29