This happened again but this time "smbd" was present among the processes in iosnoop.
Code
smbd 2911 RM 8,16 5033242544 4096 6922.81
kworker/0:1H 383 WS 8,16 3905925968 28672 0.33
jbd2/sdb1-62 622 FWS 8,16 18446744073709551615 0 70.79
<idle> 0 WS 8,16 3905926024 4096 0.22
<idle> 0 FWS 8,16 18446744073709551615 0 24.96
I wasn't using using smb at the moment and the only other device that has access to the share is my nvidia shield tv so it must be it.
"find /sharedfolders -amin -1" showed a bunch of random directories.
Code
/sharedfolders/WD-RED-Media/Serie TV/Big Mouth (2017)
/sharedfolders/WD-RED-Media/Serie TV/Mr. Robot (2015)
/sharedfolders/WD-RED-Media/Serie TV/Bates Motel (2013)
/sharedfolders/WD-RED-Media/Serie TV/Bates Motel (2013)/Season 02
/sharedfolders/WD-RED-Media/Serie TV/Bates Motel (2013)/Season 05
/sharedfolders/WD-RED-Media/Serie TV/Bates Motel (2013)/Season 03
/sharedfolders/WD-RED-Media/Serie TV/Brickleberry (2012)
/sharedfolders/WD-RED-Media/Immagini foto video
/sharedfolders/WD-RED-Media/Immagini foto video/Cellulare Ionel
/sharedfolders/WD-RED-Media/Immagini foto video/Cellulare Ionel/gennaio - febbraio 2016
/sharedfolders/WD-RED-Media/Immagini foto video/Cellulare Ionel/montagna marzo 2016
/sharedfolders/WD-RED-Media/Immagini foto video/Cellulare Ionel/romania marzo 2016
/sharedfolders/WD-RED-Media/Immagini foto video/Fotocamera Gheorghe
/sharedfolders/WD-RED-Media/Immagini foto video/Fotocamera Gheorghe/capodanno 2017
/sharedfolders/WD-RED-Media/Immagini foto video/Cellulare Alex
/sharedfolders/WD-RED-Media/Immagini foto video/Cellulare Alex/Dicembre 2015
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I think the reason "smbd" didn't show up ealier is because I run the iosnoop command after the drive woke up. I forgot to mention that, my bad.
However, I'm pretty sure I have seen the drive woke up without any activity shown in iosnoop in the past, so I'm not excluding SMART yet.