I think the SSD uses more power than the original hard drives do. I bought an adapter for the iPod classic to put an SD card in it and there was a few articles about power draw being higher for ssds. I might be misremembering the comparison though, it's been a couple years.
That may be due to other factors. I will be up front that I don't have a lot of specifics, but I do know this: in retro computing, SD is hard to implement as a replacement for old-school disks because it means a lot of overhead. For that reason, Compact Flash is still the preferred format for a lot of products aimed at that space.
Perhaps that is at play here, because increased overhead should also equal a reduction in battery life (modulo the spinning disk).
It's kind of surprising but not super surprising if you've ever seen the stock drive in them. The hard drives in the classic are tiny, 1.8inch 4200rpm units. Power draw on the drive case is half an amp at 3.3v. SSDs are like 5 watts plus whatever circuitry you need to convert the interfaces.
If I recall correctly, it was a special made spindle that could handle many spin ups and downs, and they used a massive 10 minute buffer so it loads up the buffer and spins down.
I used a modern 3000mA lithium ion battery which provides about 60 hours of music playback. That’s about 2 hours per day which is much more than I use!