What is plex? "plex is missing" replacing disk in a mirror [duplicate]

I have replaced a defective drive on mirror in Windows 2016 Server 

1. One disk from the mirror. Disk 1 status: Failed Redundancy, online

2. The new unformatted disk which is supposed to be part of the mirror. Disk 2 dynamic status : online

3. A missing disk

I tried to select the missing disk an choose "Resynchronize mirror" Nothing happened, then I tried "Reactivate volume" and I got "plex is missing"

What is plex and how do I rebuild my mirror?

The solution:

I selected the disk 1 status: Failed Redundancy by right click > choose "Remove Mirror" for the disk labeled missing, next you select the disk which already is in the mirror and choose "add mirror" a dialog box pops up and you choose the new created disk.

This solution resolved the 'plex is missing' error.