Confused and dyslexic

I need help, instructions and information seem to read as nonsense to me, so I have this simple question.
I have attached an external drive to my WD PR2100 and transferred to “public” some films to use on Plex.
Now if I disconnect the external drive will Plex have trouble indexing the films and music , or is the public path on the PR2100 a fixed point of reference for the Plex.

It is not clear what “transferred to public” actually means. The My Cloud uses Shares (which generally can be considered folders/directories). Those Shares can be set to either Public or Private. Public Shares are Shares that all can access. Private Shares are Shares where one can configure the level of permission access (Read-Write, Read Only, or No Access) and will be prompted to enter a user name and password to access. Western Digital typically has one default Share on the My Cloud called “Public” that is configured for Public Access. Within Plex you would configure which Shares (or subfolders within Shares) are used as Libraries by Plex to hold media.

If you have copied files from an external USB hard drive to an internal My Cloud Share such as the My Cloud “Public” Share then removing the external USB hard drive will not affect those copied files. Generally you will only have a problem if you have configured the external USB hard drive Share to be a Library (or part of a Library) within Plex. If you have done that then removing the USB hard drive would impact that Plex Library it is part of.

Some additional information.

My Cloud OS 5: Online User Manual
https://products.wdc.com/nas0s5/nasum/en/

MyCloud, My Passport Wireless: Best Practices while using Plex Media Server
https://support-en.wd.com/app/answers/detailweb/a_id/19078

Plex Support
https://support.plex.tv/articles/

Note that there is a specific subforum where the PR2100 is discussed and where users more familiar with that device may be able to assist with specific Pro Series My Cloud devices.

OS5 My Cloud Pro Series
https://community.wd.com/c/os5/my-cloud-pro-series/256

Thanks, I believe I have got the gist of it now, your other links are most informative thanks