I have a pretty large plex metadata folder (250GB / 1.5M files), which is currently sitting on a SSD drive using Unassigned Devices.   I recently built a new system and installed a Corsair MP600 1TB to act as a cache drive and have been moving the appdata over to it, but the issue is when copying the plex metadata folder. Before the NMVe drive I was using a 1TB WD Black HDD and copied the plex directory from it to the SSD in 7-8 hours (I think... it finished while I was asleep).