I don't have iTunes 11 (which COULD be the root of your issue) but assuming it's not...
Below assumes that you have followed Apple's instructions on how to move your iTunes media:
Now double check in iTunes prefs (advanced tab) that you've pointed iTunes to the "iTunes Media" folder on your external HD
The iTunes media Location should read something like: "/Volumes/ExternalDriveName/iTunes Media", if not, use the browse button and point it to your iTunes Media folder
If that's correct AND this is actually a permissions issue first quit iTunes then:
Do a "Get Info" on your external HD (command-I) that's not an L.
- IF: the check box "Ignore permissions on this disk" is checked, UNCHECK it.***
- OTHERWISE: under "Sharing and Permissions" at the bottom of the Get Info window it should read:
- YourUserName (Me).....Read & Write
- staff (or Admin).....Read & Write
Now, it's not a bad idea to reboot your computer at this point.
Restart iTunes, try again.
IF it still balks:
A) its POSSIBLE that iTunes Mac expects an HFS+ formatted volume (Many/most? drives come formatted FAT32)
***ALAS, if you copied your files with the "Ignore permissions..." checked, (or incorrect perm's were set) it's possible that proper permissions were not copied over.
In either event (A or B) your best bet MIGHT be to reformat the external disk (VERY easy in Disk Utility), set the correct permissions, then restore your iTunes Media folder from a Time Machine backup. (Make sure you choose the date just before