That method will erase all your ratings, play count data, and playlists! Aaaah! But I guess there's no getting around that now, since you already deleted them from your library.
For future reference, there is a way to get this done. It's tricky, but it works.
1. Quit iTunes before messing with any of your files.
2. Move your music folder to the new drive.
3. Delete them from your old drive.
4. Create a symbolic link (not an alias; see below for details) of the folder on your new drive, and put it in the same location it used to be on the old drive. Be sure it has the same name as the actual folder, e.g., "iTunes Music", not "iTunes Music symlink"
5. Relaunch iTunes, and play your songs. It should just work.
If you don't know how to create a symbolic link, download and install
SymbolicLinker. It's a CMM (Contextual Menu Module) that lets you create symbolic links of any item by control-clicking on it in the Finder.
The little gotcha with this method is that it requires that you maintain the exact same directory structure. In your case, that won't be a problem, since you're copying the folder straight from disk A to disk B. BUT, be careful not to rename any of your files UNTIL you play them at least once in iTunes. Until you do, iTunes will be able to find them ONLY by their file names and paths. After you play them once in iTunes, you can do whatever you want.