On iMacPro/High Sierra how to reclusively change permissions on an external hard disk?

1. I am working on my new iMac Pro (OS=latest High Sierra) with 1T ssd.

2. ssd on iMac works perfectly so no problem there. I did not migrate from my old Mac Pro since I wanted to sort my old work first and keep my iMac leaner.

3. All my heavy graphic work from my old Mac Pro is on a new WD-4T hard disk connected to my iMac, but without any System on it. Works great, but the 4T permissions are yet a problem (and its 3/4 full of folders!).

4. Permissions on my iMac are fine but unless I change permissions on the 4T, every time I do something, I must Authenticate or enter my pwd.

5. I now tediously change permissions on my 4T and on some of its main folders and sub-folders, but permissions on lower folders or docs do not change, even though I go through the "Get Info," add Administrator and "User=Me," and "Apply to enclosed items."

5. I have thought of using the Terminal syntax "$ chmod -R +rw" or "$ chmod -R +rwX"plus folder/dir name, but my Terminal is addressing my iMac Pro, and I may mess up my ssd by doing this.

QUESTION: How to change permissions "globally" (recursively) on my external (no-system) 4T hard disk, and if via Terminal, what is the syntax?

Much anticipated and appreciated answer. Jim