Could not unmount disk.

I have a Lacie CooKey 16gb USB key, 1 Partition as GUILD Partition Map. It was working perfectly until this morning.

*The problem:*

I can not write to the USB key and I get the following error: The operation can’t be completed because an unexpected error occurred (error code -50).

I can not delete from the USB key and I get the following error: The operation can’t be completed because an unexpected error occurred (error code -50).

I can not eject the USB key and I get the following error: The disk "####" wasn't ejected because one or more programs may be using it. To eject the disk immediately, click the Force Eject button.

*I tried to fix it by:*

A re-partition, full erase or repair using Disk Utility but it can not and I get the following error: Could not unmount disk.

I then booted up Mac OS X from the original install disk and did the same but recieved the same error.

Finally, I put the USB key into a Windows PC, it mounted fine and would unmount fine. I tried to format it to FAT32 on the PC however it would not let me simply coming up with "Error".

The permissions on the USB key are set to "Read & Write".

I am at a loss as to what happened and how to format my USB key. Please help!