Consider moving your iTunes library to the NAS drives. You’ll gain the RAID protection against drive failure.
Alternately, you can attach the USB drive to the NAS. You won’t have the RAID protection, but you will not need to leave the Mac on 7/24 so Plex can access the music.
Either way, you can mount the shared folders from the NAS on the Mac when you want to run iTunes (can automount when Mac boots if desired).
A couple of notes re: Plex on Synology.
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Do not install PMS v1.21.0.3744 that is available in the Synology Package Center. It is an old beta version Synology will not remove or update. It is not fully compatible with the current Synology OS.
Download the current version of PMS from plex.tv and perform a manual installation. Be sure to download the DSM 7, Intel 64-bit version. -
The Plex Synology FAQ has information on running Plex on a Synology NAS, including manual installations, moving from another system, etc. Definitely worth a read before installing Plex on the Syno.
