I think similar issues were discussed earlier but had different flavor...
About my NAS configuration: Debian full installation then OMV on top, system and all apps are on SSD hard drive (128 GB). Then there is a RAID5 array (4 HDDs) which is intended to save media files only.
I installed PLEX server OMV plugin and wanted to configure it running database on SSD drive but path selection was only possible towards RAID. So I went for that option as there was no other choice available. Then 2 issues arised due to this:
1) RAID never spins down, i.e. it looks like PLEX never sleeps and runs some background activities. I tried many things thru Plex configuration options but gave up when read about similar issues on Plex forums. Hard drives were kept at 41 C degrees, not least to mention power consumption and wearing issue for HDDs which may lead to shorter life time...
2) While processing metadata, i.e. creating database and thumbnails, the whole process was blody slow as PLEX was reading files and writing metadata to the same hard drives/RAID.
Then after some time I decided to uninstall PMS plugin and went for Linux way of installing the package. All went well, database is located now on my SSD system drive and media files are separated so the RAID spins down when not used, their temperature is 28 C degrees and creating database is times faster and much more quite process.
What are the reasons not allowing PLEX database location on system drive
Shouldn't admin decide where to keep database on