I have been using a Seagate Expansion USB drive as my time machine backup. Having upgraded to Lion, I found that the disk would not mount. After trying several things, I ended up erasing and re-formating.
Time machine ran well for a while.
Now this morning,it would not mount so I ran disk utility. It tried to repair the boot volumes, and failed. The drive however mounted at the end, but it is now set to read only. I have tried changing permissions in the finder, but that does not work.