'include' problem: failed opening ...

I'm having a problem using includes in PHP3. The error I'm getting is, for example: Warning: Failed opening 'test5.php3' for inclusion in test4.php3 on line 6.

It seems that when I do multi-level includes, this command fails on the 4th level of include. I wrote a series of small test scripts like so:

<?
echo "this is test4.php3";
include "test5.php3";
?>

where test1 included test2, test2 included test3, and so on up to test6. The error message reproduced above occurs when test4.php3 tried to include test5.php3. OK, I accept that using this many levels of includes may not be great technique, but the REALLY bizarre thing is that a few days ago some test code I was writing WASN'T erroring, but now it is - and I'll be damned if I know what's changed with the system environment to have caused this. We're running Apache web servers, with no other failues reported in other areas.

Can anyone shed some light?

OK, so I'm replying to myself here: we have a possible hard drive failure which may be causing the problem. Investigating now...