Just to be clear - when you say "Write more than 26GB on it", you mean:
You are trying to write files that would cause the drive to be at least 26GB full, regardless of the size of the file? (e.g., the drive is currently 25.5GB full, and you're trying to copy last night's "Project Runway" over, and it's hanging)....
As opposed to:
You are trying to copy a folder/file that contains more than 26GB in it at once.
I believe that was what Lisa Maree asked and you answered.
Since you said you had it at full capacity recently, I'm presuming you mean - you had 200+GB of data on it, and then you deleted a whole bunch, and now you're trying to put stuff on it again. Is it possible than when you deleted the files, the computer - and I can't explain why - never reallocated the space for writing? What if you run something like Eraser to clear the unused space?