Seems that there’s an occasional problem with Time Machine – today mine said that I didn’t have the appropriate access to use my external FreeAgent drive for backing up my MacBook – no problems with my iBook however!

A google search later and I found this work-a-round:

What to do if Time Machine reports you don’t have permission

This was posted in the Apple Discussions by V.K. I have only modified it slightly to be more generic.

The problem seems to be that 10.5.6 changes permissions on a file so that even the root user doesn’t have write peivileges. I have no idea why they did this. The workaround suggested in another post will probably work, too, but i did something less drastic. Instead of deleting the file I changed permissions on it, and it worked just fine. An added benefit is that the permission change seems to stick, so you don’t have to delete the file every time you change a drive.

[Open the Terminal application in your Utilities folder. At the prompt enter the following command:
sudo chmod 644 /volumes/”TM drive name”/.xxxxxxxxxxxx

The name of the file .xxxxxxxxxxxx is based on the MAC address of your computer and will be different for every computer. Put the name of the TM drive in the above and keep the quotes.

You’ll have to enter your admin password (which you won’t see) which is normal.

This worked brilliantly – took all of 5 mins and my backup is up and running again.