THE PROBLEM: Tiger's Spotlight indexing processes, when combined with Norton AntiVirus 10 Auto-Protect processes, have blocked mounting and unmounting of volumes by Disk Utility or such diagnostic/repair applications as DiskWarrior 3.0.3 and TechTool Pro 4.0.4.
THE WORKAROUND: With thanks to Symantec tech support for help. Exclude the Spotlight-V100 folder on all volumes on internal and external hard drives from NAV's auto-protect scanning. In NAV 10 Preferences -> Auto-Protect Preferences -> SafeZones, select "Everywhere EXCEPT in:" and then do this:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050802191223375