We’ve created “FAQ” threads for platform-specific info which supplement the https://support.plex.tv topics.
For Syno: “Syno FAQ” (search for it)
and you’ll find:
Scrolling down the list of tidbits, you’ll find
Take your time reading it.
There are a lot of steps but there are also screenshots 
to help.
ASK if you have questions.
To give you an estimate of the time required:
- On single disk volume (JBOD) systems: 3 minutes per 1000 items
- On multiple disk volume (SHR/RAID) systems: about 2 minutes per 1000 items.
You will hear it rattling the drive(s) when this is happening… It’s normal.
