time machine seagate expansion USB drive has gone read only

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.

Any ideas??

Jonathan