External HD spins up again after eject

The script you are referencing was made a long time ago and it parses the output of the utility "hdutil". I'm sure the output of that command has changed over the years so the parsing will give incorrect results today. You should be very careful running any Terminal commands without understanding exactly what the commands do especially commands written for another major version of macOS (even if they are separated only by a year). You are lucky the command didn't cause any damage.

Try connecting the external drive directly to the Mac to eliminate any issues with a hub. If the drive is connected directly to the Mac, then try using a different USB port, a different USB cable, and maybe try using a well respected brand of powered USB 3 hub with UASP support which might help insulate the drive from the Mac. I've had issues with external USB3 drives when connected directly to a Mac.

It is also possible you have an issue with the external drive itself. Many external drives are cheaply made and are not always of good quality so they can have compatibility issues.

Edit: If the drive is making a scraping sound, it may indicate a failing drive. You do have to be careful not to move a hard drive while it is still spinning (or at least move it slow & gentle) or it can damage the drive.

There might be some variables. I have an OWC USB3 dual dock for hard drives:

In another bay there is a 3TB HD partitioned into three, and when I unmount them all (Command-E), the volumes unmount, and the drive spins down-up three times, until finally it spins down.

In another bay there is a 1.5 TB HD partitioned into two, and when I unmount them (Command-E), the volumes unmount (*), but the drive keeps spinning until I cut its power. (*)Usually the other volume holding the (fully indexed) Photos library is reluctant to unmount and sometimes it must be force-ejected. Maybe the photolibraryd still wants to inspect the photos.