MySql Error 28 after upgrading to PHP 5, MySql 5

[damon]

I'm getting these error 28's across several sites, very confusing since I only have one "device", and it has 120gb of free space.

My setup is a
1and1 business server 1 built with Centos 4.4
Plesk 8.1.1
mysql 5.0.27
php 5.2.3
Apache 2.0.52
Linux 2.6.9-42

I've researched this error and it always comes down to running out of space in the MySql temp directory. But I don't know how this should be happening since I have one big partition. Another weird thing is that I can't seem to change the tmpdir location using my.cnf . I've added
tmpdir=/var/mysqltemp and restarted mysql but when I check the variable

mysql> show variables like "tmpdir";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| tmpdir        | /tmp/ |
+---------------+-------+
1 row in set (0.00 sec)

I hope someone can help, pretty much all the sites on this box depend on MySql, and I'm about to launch a big upgrade of the busiest site on the server, which is all database driven.

Anyone have ideas? Or if you are or know a real MySql setup guru I would like to hire someone to really optimize the server for database operations.

Thanks,