Normally you would want to use the Remove Drive feature to disconnect the faulty drive from the pool. However, I've seen instances where this was very difficult to do; it would either take an enormous length of time, or fail completely.
The most important thing, I feel, at this point is to ensure you've got everything backed up as much as possible.
Another thing you can try, if you're prepared to leave the server shut down for a day of two, is to get a copy of SpinRite and run it against the faulty drive. But, I would do this after running CHKDSK on the drive, in case it's just a file system issue. That seems somewhat unlikely at this point, but you never know.
In the end, you should be prepared for the situation where you have to completely rebuild the Data Pool and copy all the data back to it from a backup.
Thanks for the advice Ikon.
That's what I'm doing now - I'm using a live boot Disc to copy the data to an external drive. Of course my concern is the other 1.5TB drives could fail as they were purchased all at the same time.
I've removed the faulty drive and connected it to another system and used "GetDataBack" to access the drive. Sadly it looks like the drive is faulty.
I'm already preparing to rebuild the pool but I'm thinking I might take this opporutunity to migrate over to 2011 and build a new box.
Unfortunately there is no other backup so any data thats lost is gone forever but luckily the only folders that weren't set for duplication are movies I can re-rip.
The whole situation is a pain but it my fault for not enforcing a better backup proceedure. This time around I'm making sure they have a backup!
Thanks everyone for all your help, its good be part of a community