I will add the suggestion to use Task Manager, Resource Monitor, or Process Explorer (Microsoft, free) to look for what all is running when the drive is in use.
https://docs.microsoft.com/en-us/sysinternals/downloads/process-explorer
Use all three tools but only one at a time.
See what you can find, if anything, when the drive is doing unexplained reads/writes. Could be some background process that gets launched at startup or perhaps later being triggered via Task Scheduler.
If you can identify the culprit (an application or utility for example ) that is known to you - then disable it.
Otherwise, simply post what you find or anything else that seemed odd or astray when the above tools were run. For example some application that you do not recognize.
It can be risky to just disable or otherwise stop a process that is not known. Unknown processes can be researched to discover what they are and what they may be doing or trying to do.
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Also: do you have all important data backed up to other locations? Just in case the NAS/hard drive fails.....