Patrick O'Callaghan:
Interesting idea. Mine is this model:
https://www.amazon.co.uk/gp/product/B06XYJGDTH/ref=ppx_yo_dt_b_search_asin_t...
The power block says its output is 3A at 12V.
The drives are both WD model WD10EZEX, (though the label on one says it has a 64MB cache and the other doesn't). Both labels say 5VDC, 0.68A and 12VDC, 0.55A. Looks like the dock's power should be enough.
George N. White III:
The spec sheet https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/western-digital/product/internal-drives/wd-blue-hdd/product-brief-western-digital-wd-blue-pc-hdd.pdf says 12V peak load is 2.5 A. It does take power to spin up to 7500 RPM, and I doubt your supply could survive without staggered startup.
Or, the drive could do power management (if they were designed better). i.e. Not try to start the motor at full pelt, but ramp it up gently. It's not just those little docks' power supplies that might badly hiccup under sudden load.
30 seconds does seem a bit of an extreme length to delay things, though. We're not talking about the old washing-machine sized hard drives with huge platters.