external hard drives won't sleep

SoftRAID does not manage sleep for disks, that is completely under the control of OS X.

I can't think of anything that would change the behavior of multiple disks to not sleep, except spotlight/indexing. If you have a large volume, it could be indexing a lot more. But that should dissapate.

Check that you don't have "wake for network access" enabled, that can cause the drives to stay spun up.

Just got a Thunderbay 4 setup in RAID 0. Was expecting that the device would sleep when the computer sleeps as that is OWC's claim in documentation. Unless my iMac (late 2013 running 10.12.6) is actually turned off, the array stays on and the fan runs.

I contacted OWC support about this and they told me that the issue is with SoftRaid having access or monitoring the drive at all times. If any sort of data signal is sent over Thunderbolt the array won't sleep. Obviously that contradicts your statement above so trying to get to the bottom of this.

I've tried changing energy settings in system preferences. Also tried turning off all monitoring functions of SoftRaid to no avail (although I did not restart after). Can't think of anything else to try on either side. Thanks.

SoftRAID does not do any activity that would keep drives awake. SoftRAID has no control over this, we completely defer to OS X for sleep.

This is an OS X issue.
Here is a workaround: If you unmount your volume, then the drives will sleep.

We need to report this to Apple as a bug. All drives should sleep when the system goes to sleep. Whether they fix this is another question.