Boto: a small file upload works, but a large file doesnt

Question

I have a script that works great when presenting the user with a list of files stored in the S3 bucket, which, when they select, loads and does something with the file.

This method works with files up to 600 MB, but when the user selects another file, which is 2Gb, it receives a Boto exception error indicating that the file is being used by another process.

listname = self.list_ctrl.GetItemText(i)
conn = boto.connect_s3(access_key, secret_key)
bucket = conn.get_bucket('data')
key = bucket.get_key(listname)
key.get_contents_to_filename(key.name)

This is really puzzling, as it works great for small files.

Any ideas what could lead to its failure?