Hi. I found that Yosemite seems to have an issue with permissions when rebuilding the Apple Loops Index for Logic Pro 9. I also tried to rebuild the index by dragging the apple loops onto the loops browser only to have it prompt me for an Administrator password but not doing anything. Here's how I fixed it:
Delete the existing index files:
1) Quit Logic Pro 9
2) Delete the *.txt files in the /Library/Audio/Apple Loops Index/ folder
Add yourself with read & write privileges for the loops and index folders
3) Right (Control) click the /Library/Audio/Apple Loops/ folder and choose Get Info
4) Expand the Sharing & Permissions in the Get Info window
5) Click the Padlock icon so you can make changes. You will likely be prompted for your password
6) Click the + button and choose your user account (name) and click the Select button
7) Make sure you privilege is Read & Write
8) To apply the permissions to the sub-folders and files, select your username from the list, click the gear icon and click "Apply to enclosed items..." Review the warning.
9) Repeat steps 3-8 but this time for the /Library/Audio/Apple Loops Index/ folder
Reindex as normal
10) Launch Logic Pro 9
11) Open the loops browser
12) Drag the /Library/Audio/Apple Loops/ folder into the browser
13) Enter the password for your login as prompted. This time you should notice it will take a lot longer. Once completed, your loops should work fine. This worked for me. Hope it helps.