First you need to get lmce to see your files. In the web admin, open Files & Media -> Media Files Sync on the top menu. Now, on the left side you should see a folder structure. Here you should see your disk also.
I haven't used external or separate internal disks, so I don't know for sure, but I think lmce should detect them automatically, and present you with an option what to do with them.
You will find more information about this if you search the forum or the wiki.
When/if you see your drive in the left structure, navigate to the folder in question (all using the left structure). When you select a folder, you will see its contents on the right. In this right pane, you have a option to "Edit attributes for all files in this directory". Using this, you can edit attributes for all files in that directory. In 710 it is not possible to recursively set attributes in subdirectories, but I think this will be added in 810.
These subject has been discussed a few times already, so I am confident you will find better explanations elsewhere in the forum ;-)