I use a Synology NAS and had this problem a very long time ago. I slowly disabled everything until eventually I got it to the point that the only thing running was the file manager and file network shares.
There is a load of things installed by default which you don't need... disable and delete things.
Mine's currently a single 8TB drive but I've just liberated a 1TB SSD from the PS4 so thought I'd look into installing the OS on that. Assuming it doesn't make any difference to the disk access issues I'll give your suggestions a shot.
It's the search indexer or the music service.
I use a Synology NAS and had this problem a very long time ago. I slowly disabled everything until eventually I got it to the point that the only thing running was the file manager and file network shares.
There is a load of things installed by default which you don't need... disable and delete things.
Also... https://synocommunity.com/package/synocli-monitor will do what you want. Install and enable SSH and then use standard Linux tools to see what is happening.