Because you are an Apple OS user, you probably know better the dissimilarities compared to other operating systems in the world, so have a closer look into the direction, what may be different between a disk volume on a Windows machine against a disk volume on an Apple machine.
Do you try to rename files on a HFS+ volume, which has been formatted as significant case sensitive, in order to allow uppercase and lowercase letters to be different letters?
If so, then you have to observe this case.
This can also happen on a SMB network folder.
It could be, that ...
"Joe Smith - A Great Day At The Beach.mp3"
"Joe Smith - A Great Day At the Beach.mp3"
... are two different files within the same folder on the disk.
This way it is clear, why the renaming fails.
See book "Mac OS X Server Essentials".
See also ...