To recap:
LibreELEC 10.0.4
System Tools 10.0.0.134 from Team LibreELEC
Raspberry Pi 4b 4GB
Seagate Expansion STKP14000402 14TB
USB drive spins down once, but after spinning back up, the drive's LED is out and it won't spin down again until a reboot.
As per elonesna's example, I modified the command in /storage/.config/autostart.sh to match my configuration:
"/storage/.kodi/addons/virtual.system-tools/bin/hd-idle -a sda -i 300 -l /storage/hd-idle.log"
With the Seagate Expansion STKP14000402 drive, behavior was unchanged from my previous commands. (It spins down once, but after spinning back up, the drive's LED is out and it won't spin down again until a reboot.)
I decided to shut down and swap a 2TB Ext4-formatted WestenDigital drive for the 14 TB ExFat-formatted Seagate. I didn't change anything else, and the WesternDigital drive spins down, back up, and down again as expected. Its LED also works correctly after spinning down and up multiple times.
Working properly with one drive and not the other makes me think the configuration is basically correct, but there's an incompatibility with that Seagate drive. Commands tried-
"hd-idle"
"hd-idle -i 0 -a sda -i 300"
"/storage/.kodi/addons/virtual.system-tools/bin/hd-idle -a sda -i 300 -l /storage/hd-idle.log"
Here's the complete hd-idle.log contents, annotated as to which drive was connected:
Seagate
date: 2023-07-08, time: 18:51:35, disk: sda, running: 330, stopped: 61
WesternDigital
date: 2023-07-08, time: 20:51:54, disk: sda, running: 300, stopped: 511
date: 2023-07-08, time: 21:39:55, disk: sda, running: 301, stopped: 2580
date: 2023-07-08, time: 22:07:58, disk: sda, running: 302, stopped: 1381
Thanks again for the help-