I had update problems with my new F12 install, but was able to sort things thanks to a tip Leigh posted in another thread. If you haven't already tried it, run :
It will take a while, even with presto reducing the download size, but after running it subsequent updates have gone smoothly.
Hope this helps.
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Desktop #1 F18
Desktop #2 Mint 14
Laptop: Macpup 529
Netbook: Debian ARM