I have a WD my book external hard drive plugged into my imac and I want to share it over my network with my macbook pro. I went into system preferences > sharing, checked "file sharing" and added my external hard drive to the shared folder. So now it is listed in "shared folders" but under "users" everything is grayed out (it says "read and write" for "everyone" - but "read and write" is grayed out, and it doesn't let me add or subtract users). Now I can access all of the folders I shared from my imac except for the external hard drive. What's going on?