I think there are a couple of problems (I am speaking in general/Linux) terms, I don't use a Mac, but -
The problem you are experiencing is you have mounted the drive as owned by user "debian-transmission". As you are not root (We know this because of the "sudo" at the beginning of your mount command), you can only edit files you own or have rights to.
The "root" user for the Raspery Pi is not relevant - all that is relevant is the UID (and GID) of the file, which is a number - root having a UID of 0 and GID of 0 (on your MAC as well as on the PI)
- See if the password you need to mount the drive is the root password for your MAC. (This is a guess, again, I don't have a MAC, so I don't know what its doing in the background. The password will either be for root or for "debian-transmission" which does not exist)
or (probably better)
Have a look at the ownership of a file you created (ie the username - which would be in place of "debian-transmission") - and execute the sudo mount command but replace the "debian-transmission" with your username.