I have same situation, when I get time I will work on that.
For now, what I do is go through a terminal as root, and use command:
mv -fra filename /mnt/data
The -f is for force without confirmation (so I don't have to hit (y) to confirm for each individual file.
The -r is for recursive, for a folder/directory, to make sure everything within gets moved also, if applicable.
The -a is for all files including hidden files if applicable.
Bash will tell me it failed to preserve permissions/ownership, which is not a big deal for me.