I have a couple of disks that I use as offsite backup drives that I only want to be complete copies of my primary backup drive. None of these are drives I want to have Spotlight indexed per se, but from time to time, I might need to recover a file. So after experiencing abysmal wait periods when I was doing a Spotlight search (since the external drives have to spin up before delivering anything to Spotlight I guess), I added these to the Spotlight Privacy preferences.
When trying to recover a file later on, I used the Finder's search field as I have always done on Panther. However, even though I was doing a search limited to one folder tree, I was not getting any results. That's when I discovered that the Finder search field, at least for local drives, only searches using Spotlight -- whether or not this is enabled for the drive/folder or not.
Currently I don't know of any workarounds either, except going command-line...
[robg adds: I hadn't noticed this problem, either, until reading this hint. After testing, I confirmed that, if you have a drive that Spotlight is set to ignore, you can't find anything on it at all. It seems that a basic Finder search should be possible. With the Privacy tab, though, it seems it's all or nothing. Perhaps a way to set "Allow Finder searches by filenames" for drives marked Private. The terminal is the only workaround at this point (via locate or find . -name ...).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050601125353242