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TL;DR: Unraid's behavior with enterprise-grade disk is a little odd, but I managed to make those drives to sleep, and slightly patched the GUI to display the real state of disks.

It turned out that running hdparm -S wasn't enough to COMPLETELY prevent disks wakes-up, but reduce it to once every 4-5 days. Which is satisfying enough to keep this configuration.

In the end, here are the mandatory configuration items :
Set Default spin down delay to NEVER. It is absolutely crucial to set the default to never ; otherwise Unraid keeps waking the disk every 9 hours (even if the default is set to X minutes, and specific disks are setup to never be spun down). For disks that aren't Enterprise-grade HDD, set a specific spin down delay to whatever.
APM must be disabled (hdparm -B 255 /dev/sdX)
Smart offline data collection must be disabled as well (smartctl -o off /dev/sdX)
EPC doesn't seem to have any impact
The only caveat of this setup is that from the GUI, disks are always marked as "Active" ; even if the disks are spun down. I think it is related to the fact that the default spin down delay in Unraid changes the global behavior of the disk monitoring. Idealy, to fix this issue, we would have to gain control over what writes into /var/local/emhttp/disks.ini, but this file is being overwritten every second or so by a thread of emhttpd.

What I did is setup a patch for /usr/local/emhttp/plugins/dynamix/nchan/update_2 ; checking for the state of Toshiba enterprise-grade disks. For those interested, here is the patch :
--- /usr/local/emhttp/plugins/dynamix/nchan/update_2    2026-02-20 16:46:42.299061313 +0100 +++ /usr/local/emhttp/plugins/dynamix/nchan/update_2.patched    2026-02-20 16:48:55.535689177 +0100 @@ -169,6 +169,9 @@   function device_status(&$disk, &$error, &$warning) {   global $var; +  if (str_contains(_var($disk,'id'), 'TOSHIBA_MG') && str_contains(exec('hdparm -C /dev/' . var($disk,'device')), 'drive state is:  standby')) { +    $disk['color'] = 'green-blink'; +  }   if (var($disk,'type')!='Extra' && var($var,'fsState')=='Stopped') {     $color = 'green'; $text = 'off-line';   } else switch (var($disk,'color')) {To apply it immediately, run patch /usr/local/emhttp/plugins/dynamix/nchan/update_2 < /boot/custom/update_2.patched and restart nginx (/etc/rc.d/rc.nginx restart).
To make it permanent, modify your /boot/config/go file to patch on every boot.