Postgres "could not write to hash-join temporary file"

[rangi500]

Hello,

We've recently started getting errors like this:

Last error:
ERROR: could not write to hash-join temporary file: No space left on device

But there is plenty of space on the all the drives.

Does anyone know what might be causing this?

My postgres version string is:
"PostgreSQL 7.4.6 on i386-redhat-linux-gnu, compiled by GCC i386-redhat-linux-gcc (GCC) 3.4.3 20041125 (Red Hat 3.4.3-6.EL4)"

Any help much appreciated,
Rangi