The Macintosh HD icon became less visible a few years back, when new macOS installs stopped showing your internal disk on the desktop by default. It has also been many years since Apple shifted to SSDs as the primary boot media for new Macs. It's not clear why the icon is being replaced now, instead of years ago -- maybe the icon had started clicking, and Apple just wanted to replace it before it suffered from catastrophic icon failure -- but regardless, the switch is logical (this is a computer storage pun). Apple's iconic Macintosh HD hard drive icon was first introduced in a 2000 Mac OS X beta and remained largely unchanged for over two decades, with only subtle updates in 2012 and 2014.
The first SSD-equipped Mac was in 2008, "when the original MacBook Air came out," notes Ars. "By the time 'Retina' Macs began arriving in the early 2010s, SSDs had become the primary boot disk for most of them; laptops tended to be all-SSD, while desktops could be configured with an SSD or a hybrid Fusion Drive that used an SSD as boot media and an HDD for mass storage. Apple stopped shipping spinning hard drives entirely when the last of the Intel iMacs went away."
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