I'm not siure, but I'd try it, Find & Spotlight are halfway tied together I believe.
A couple of other options for searching...
Here's a direct download link to EasyFind 4.0 which runs in 10.3.9 & up...
From this page,
* EasyFind 4.0
* PhotoStickies 5.6
* ThumbsUp 4.3
* XMenu 1.8
Use it to search your whole drive for case insensitive & show invisibles...
(dot ee em el ex)
Find Any File...
Hold Option or alt key when selecting Find to Find All.
You want the old drive around for safety but leaving it mounted means that you (or any app) could delete or change items on the disk.
Wouldn't it be more sensible to remove it? Failing that create an Automator script to unmount the disk on login - at least you won't accidentally edit the disk that way. It can be re-mounted in Disk Utility.
Personally I tend to remove drives from these older Mac Pro's, it's too easy to shut down & slide out the HD. You can store it away from the Mac protecting you a little in the case of theft.