Disk that exists in fstab was mounted by thuar to /media/mountpount as in fstab. In this moment thunar doesn't allow to modify files, but when open this dir by terminal i can do anything: creating/renaming folders, etc.
Please provide more information: 1. What type of mount is this (i.e. nfs, cifs, local disk)? 2. What type of filesystem (note that this may be implicit in the answer to #1)? 3. What does "ls -ld" of the toplevel mountpoint show? 4. What does "ls -l" of the mountpoint show? Feel free to trim or redact any sensitive filenames - I'm only interested in the permissions and ownership.
Unfortunately, it was long time ago and I can't reproduce the bug anymore. So I think this bug report can be closed now.
Closing per reporter.