Spotlight won't search external HFS drive (permissions issue?)

I have a brand new Toshiba 2TB external USB drive that's fresh out of the box as of two days ago. I manually formatted the drive using Disk Utility to be Mac OSX Extended (HFS). I then copied about 500 GB of files onto it. Yesterday, I was able to use spotlight to search the drive. Today, spotlight gives no results for that drive (all internal drives OK). It's connected to the same computer and I've made no changes to my OS.

Running a MacPro 1,1 with OSX 10.6.8.

I have tried the following:

- Open System Preferences > Sportlight > Private; drag drive to list (with intent to then remove it in order to force re-index). When trying to add, I receive an error: "Privacy List Error: The item couldn't be added or removed due to an unknown error."

- Rebuild file directory in Disk Warrior. I receive an error: "Directory cannot be rebuilt error code 2153, 4903."

- Fix file permissions for the disk: when I try to add "Administrators" to the name list at Get Info > Sharing and Permissions, I receive an error: "The operation can't be completed (error code -50)."

I can see all the folders and subfolders on the drive and click through them by hand, but I can't search.

Also, when I try to manually rename any folder on the drive, I receive an error (OS thinks I need fewer characters or no punctuation, but a simple folder name like "Folder1" is not accepted).

I suspect this drive has developed a permissions problem somehow in the last day or so.

How can I fix this without having to copy everything off and reformat the drive again?

Thanks!