[CentOS] MySQL error 28, can't write temp files - how to debug?

Hi all,

I hope someone can help with this one.

On one of our VPS's we have a fairly large MySQL database, which gives me the following error when running some queries against it:

An error occured!Can't execute statement: Error writing file '/tmp/MYL4qeT5' (Errcode: 28) SQL: , , select * from tips order by rand() limit 0, 1 ,

According to google search, errorcode 28 means the HDD is full. But it isn't:

root@vps:[~]$ df -h Filesystem Size Used Avail Use% Mounted on /dev/sda1 84G 18G 62G 23% / none 640M 0 640M 0% /dev/shm /usr/tmpDSK 485M 11M 449M 3% /tmp

What else could cause this kind of problem?