Verify volume failed with error Could not unmount disk
 

I hope this is the right place to request help with the error message: "Verify volume failed with error Could not unmount disk" I use a Mac OS X/10.4.11/Dual 2 GHz PowerPC G5/ 7 GB DDR2 2SDRAM. Software is fully updated. The computer has two internal disks and an external 1.5 TB FreeAgent...