I just did something similar on Windows.
I used Windows Explorer and moved the folder and then created a Windows Junction or hard link. You need to stop any program that is accessing the folder that you want to move.
I don't know what folders/drives you have, but should be something like this:
If the folder occupying all the space was located at
Move mydata to J:\Ethereum\mydata
Now create a Windows junction (or hard link) so that whichever program is trying to find MyData will find it at its new location.
The junction has to be created in the folder where the data used to exist.
2a. Open a Command Prompt as an administrator
2b. Navigate to C:\programfiles\ethereum:
2c. Now create the junction:
mklink /D mydata J:\Ethereum\mydata
2C. Here is the same as above, just trying to make it a little cleared:
mklink /D C:\programfiles\ethereum\mydata J:\Ethereum\mydata
Now create a random text file inside of J:\Ethereum\mydata
Then navigate to C:\programfiles\ethereum\mydata and make sure that you can see the text file.
If you can see it, you are done.
Just simply start the program, and it will start using the new location without knowing it was moved.
The junction is also permanent, it will stay until you delete the folder (junction) that you created on the C: drive