Use -B instead.

Sets Advanced Power Management feature. Possible values are between 1 and 255, low values mean more aggressive power management and higher values mean better performance. Values from 1 to 127 permit spin-down, whereas values from 128 to 254 do not. A value of 255 completely disables the feature.)

I never got hdparm to work for me either. Hd-idle however works like a charm.

Besides the caveats about disks living longer if they are kept spinning,

I think that's not necessarily true. I think spinning 24/7/365 has its downsides too, higher temperatures and others I'm less certain about.

are there reasons why I shouldn't setup a cron job (well, a systemd timer) that runs hdparm -Y every 10 minutes? (for example, could hdparm -y cause errors if run while the drive is being backed up?)

because you'll often shut it down while someone would be using it. and then it can spin up immediately. the processes accessing it would probably hang for half a minute or such.

there is a better solution, hd-idle, as said in the other response

Does the job get suspended if tasks are being done on the drive or will the task stall with the command being fired then the drive wakes up again defeating the purpose of the job?

When you spin up the drive, the motor has to overcome the mass of the disks to bring them up to speed, requiring more torque, current, and wear, than just keeping them at that speed. On the other hand, bearings don't wear out at zero RPM. Bearings go, motor goes, either way drive is dead. Regarding bearings ALWAYS mount drives so that they are horizontal, this results in minimal bearing wear and load.

Hey I see that you found hd-idle.

Last time I tried to use it it wasn’t compatible with smartmon.

I would take smartmon over spin down every day of the week.

I’m gonna go out on a limb and assume the reason you have this problem with power is that you’re running a zfs/md-raid 5 or whatever. It may be a good idea to get away from that configuration. When you write a file to a 3 disk pool with parity, all three drives spin up. The details are a whole nother can of worms but the way that operating systems, busses and hbas interact with disks make this even worse.

I got away from that situation with a mergerfs/snapraid setup where my disks are jbod and writing a file to the pool only spins up one disk (with some caveats) and parity calculation is done at night as a snapshot all at once.

I do not think this saves power, although my power use has been very low in this configuration. I do think this saves drives, because snapraid is decently judicious with spin up/spin down work, amassing all the changes first then calculating what to put where and doing that all at once.

If your primary concern is power use and hard disk life, consider a ssd pool. The density and power consumption are why datacenters switched to them and why 3.5 racks are so cheap now in comparison.