One of my partitions behaves most strangely : writing to it is slow and causes the entire system to stutter : KDE, the console, everything suggests that the system is under IO stress.

What is strange is that it happens only when writing to /dev/hda4 ! Writing to any other hda partitions works fine and stutter free as does writing to the other disk on the IDE bus (hdb).

I thought that repartitionning it to ext3 (from reiserfs) using yast would somehow reset some counters but that didn't work.

There is nothing in /etc/log/messages.

I am baffled and annoyed at that I can't use these 20Gb. Please someone, help me !

Not a direct answer, but may help you get more forensics...

Do you have 'sar' handy? (in /usr/bin/sar on my distro) "sar -d" (if your distro sets up crontab entries to run sar data collection periodically) will show you disk i/o info.

You can also get realtime sar data with "sar -d <interval> <count>". eg, 60, 1sec bins of info... "sar -d 1 60". Run that command, then do whatever makes the disk thrash and slows your box...

Sar can also monitor CPU usage, swap usage, etc...
-c