[X4U] "Startup disk almost full" alert (10.4.11)

[LA Licata]

I started recently erasing free disk space with a single pass on my  
internal drive. The why I started is not important.

After the first pass is done, an alert that is worded something like  
"Your hard drive is almost full. You should erase some  
files" (probably not the exact phase, but close)  pops up.

I am on a 17" PB, 1.5 Ghz power pc G4, 1 GB DDR SDRAM, 10.4.11, all  
software up-to date. My TOSHIBA MK8025GAS 74.53 GB drive has  31.77  
GB free out of an available 74.41 GB. Percentage wise, 57% of the  
drive capacity is used.

During the free space wipe process, there appears to be a temp file  
written which seems to added about 4-5% to this number. Within  
minutes of the first pass being completed, and after this alert pops  
up, this temp file appears to be deleted as the indicated drive space  
used drops back down to 57%.

So, could should one suggest why I am getting the "my drive is almost  
full" popup?

Thanks for listening.

Lee