Few days ago I wrote about this problem (http://lists.centos.org/pipermail/centos/2005-June/007086.html). Now, after upgrade I have CentOS 4.1 and problem still exist.
What happens if you:
tail -f /var/log/messages
then plug the drive in? Do you see any associated messages?
Below is a fragment of /var/log/messages
[... cut ...]
Well, you pendrive is getting detected without a problem, it seems.
Maybe it is just a Gnome problem about automounting it.
Try: # mkdir /mnt/pen # mount /dev/sda1 /mnt/pen
And see what happens.
I have no such device - sda1. Thre is no any sdx devices :( It is strange.
It happened, as I wrote before, after last 4.0 kernel update.
# modprobe sd_mod
Then check "dmesg" and /var/log/messages, and see if you get anything.
Here is dmesg after "modprobe sd_mod" (http://www.ch.pw.edu.pl/~dominik/dmesg), and here is /var/log/messages after "modprobe sd_mod" too (http://www.ch.pw.edu.pl/~dominik/messages).