Juli Clover at MacRumors spotted the change:
The existing icon still resembled a hard disk drive, but the new icon looks like a modern solid state drive. Apple’s Macs stopped using hard disk drives starting more than a decade ago. The low-cost 21.5-inch iMac was the last Mac that had a hard drive component, as it used Apple’s SSD + HDD Fusion Drive. All current Macs use SSDs.
I will miss the old icon, but as Clover points out, spinning drives are long gone for most Mac users. The new icon feels more modern and fits in much better with the rest of Tahoe’s icons:
I’m just glad Apple kept Macintosh HD around.