“hdparm -S” does not work on external HDD's. How to avoid it from spinning down?

Apparently, hdparm only works with internal HDD's.
How do I avoid an external HDD from spinning down after half a minute? One way is to read one sector using badblocks or dd each 29 seconds in an infinite loop. Is there another way to keep the HDD spinning? Edit: SMART is also not supported by that HDD. hdparm -y also does not work, but works for a different external HDD.