I am planning a similar setup and I am glad to hear it worked for you. However, I don't think I quite follow exactly how you configured TM to simultaneously backup your internal HD and your external HD (iTunes, iPhoto libraries). Also, I am unclear as to whether all files on the external HD are backed up or only the iTunes and iPhoto libraries.
Is the key to a succesful configuration simply to keep the iTunes and iPhoto libraries on the external HD or will you have to "teach" TM how to behave in the case that the external drive is attached when backing up? Is this question clear?... I find that I'm having a difficult time putting thoughts into words.
Thanks!
Edit: I just wanted to add that this forum (Apple TV) seems to be the wrong place for this question. I only used the best thread I could find on the topic using the search.
It's easy to do.
First hookup and configure the external drive you want DATA ( iTunes / iPhoto ) to reside on. Once you have this drive setup and CONFIRMED the libraries are configured, you will want to removed any libraries from you internal drive.
MAKE SURE YOU CONFIRM THAT ITUNES AND IPHOTO ARE IN FACT SEEING THE LIBRARIES ON THE EXTERNAL DRIVE BEFORE YOU DELETE THE LIBRARIES FROM THE INTERNAL DRIVE. IF YOU DO NOT YOU COULD LOSE ALL YOUR LIBRARIES
The reason for removing them is to prevent Time Machine from backing them up twice.
In TIME MACHINE you cannot teach it what to backup, it will backup EVERY DRIVE attached to the computer. The only thing you can do in Time Machine is teach it to NOT backup a given drive.
NOTE: Time Machine cannot backup the drive it resides on. For this reason you will want to dedicate a drive to Time Machine and do not put any data on it that you cannot afford to lose.
In Time Machine Preferences you have 2 options:
1) Do Not Back Up This Drive ( this is where you add or subtract a drive NOT to be backed up - you will see Time Machine on the list and it cannot be removed.
2) Warn when old backups are delelted. ( this is a radio button you check ON if you want to be warned - OFF if you do not want to be warned.
My configuration has changed ....
320 Gig HDD Internal HDD
750 Gig HDD External iTunes/iPhoto
1 TB HDD Time Machine
500 Gig HDD External Scratch Drive - Not backed up by Time Machine