To repair a disk:

Quit OnyX.

Insert the Mac OS X Install disc. Restart the computer. Hold the C key after the start-chime (thus forcing a restart from the system DVD).

Select your language.

In the Utilities menu, choose Disk Utility.

In the left column, select your startup disk.

In the S.O.S. tab located at right, click the Repair Disk button and wait.

Then restart normally...