[Freesurfer] cannot save edited main volume

Hi:

When I finish white matter editing and try to save the edited main volume by "Save Main Volume" of the "File" menu on TKMedit Tools window, there is a dialogue asking me:"Are ou sure you wish to save the changes to the main volume". When I clicked yes, error message appeared as follows:" Error. Couldnt save volume. This session was started in read-only mode. you cannot save changes."

I changed my file permission to 777 (but I forgot to check my original permission settings) and saved again but in vain. Why was this error message? What should I do for now to avoid losing the edited WM results?

Thanks for your help.

Best,

Wei-Ta Chen