Same tree structure? I use SyncBackSE to compare and copy up to terabyte volumes. It's really cheap for home use to: $25 for a 5 computer license. The free demo will do what you need too.

Vice Versa is another one I've successfully used in production but SyncBackSE has more active development and the developers have an active forum.

Any diff tool will work (I use the one from http://unxutils.sourceforge.net/ since I'm on a Windows machine).

diff -qr dir1 dir2

should output something like:

Only in dir2: a.txt
Only in dir1: b.txt

I guess I'm confused, are you trying to consolidate the artist folders while copying? If not and you're just doing a straight up copy then read on.

I run two scripts, one uses Robocopy to do the actual copy. There are switches to copy just changed files after the original copy for use when doing a migration from one server to another and other neat stuff (like retry to copy failed files). I have it dump the results to a log file that looks like this.

I then run a script that uses Diruse.exe (a resource kit tool) that spits out a .csv with just summary info of the source and destination sizes of the directories I back up. Using the logs from the robocopy I can find any files that might be off.

Let me know if you want copies of the scripts.

In that case I would start a robocopy from the second drive to the nas.

I use these options with the xp/2k3 version (XP010) of robocopy:
/E - copies all sub directories, even if empty.
/NP - No progress indicator.
/FFT - Assumes FAT, helps with some file types on NTFS volumes.
/TEE - Displays to the output window and/or log.
/COPY:DATSOU - For NTFS volumes copies all the attributes, ACLs, auditing, ownership and fun stuff like that.
R:1 - Number of retries on failed copies, the default is 1 MILLION.
W:1 - Wait time in seconds to wait between retries, the default is 30 seconds.
/LOG+:\\server\share\file.log - The log location. I actually use a variable for the log location since it's dynamically named based on date/time.

If you log the copy process when done you can open it in your favorite editor (or notepad) and search for fail and see what files failed. The thing is the copy won't stop on a failure like with the 'normal' copy command so you're not wasting time restarting everytime you find an F'd up file.

Do you have "'s around the entire path?

"G:\The Associated Group MP3\MP3\*.*"

They're usually required when you have folder names with spaces. Or you could just rename the folder without the spaces for the copy, then rename it back when it's done.

What's the full command line you're using?

Here is my script:

My Shares.txt file just has 1 line per top level folder on the source drive that I want to back up, in my case:
Users
Shares

You can see what the command passed to the CMD Line is by changing the Robocopy line to this:

And that will spit out something like this:

Keep in mind these are single lines but have been wrapped by the page.