Don't think I'd ever BUY HDD's for this but I have them already now.
32 TB of drive is slightly MORE expensive (at NewEgg pricing on the Seagate Externals) than most GTX 1080 ti cards - which pull in close to $3/day each at this point.
Tradeoff is the ELECTRIC usage of that 1080ti will be in excess of 160 watts and more likely close to 200 watts to achieve that income level, while the 4 hard drives are eating around 50 watts AT PEAK and quite a bit LESS most of the time.
Oh come on. What 3$/day are you talking about? With HDD you don't have many options. Burst, Storj, Sia ... that's all we have at the moment. The only "profitable" is Burst if ROI of 2+ years can be called profitable.
With GPU you can mine everything you like. 3$ days make 1080 not 1080ti. There are of course people who mine at nicehash ... ok let them mine.
At the moment HDD mining is completely unprofitable.