I’m guessing it has more to do with the method the data is read but I’m not technically savvy enough to understand the difference.
In: Technology
It does have to do with how the data is read, but less so than you would probably think.
Optical discs, more often just referred to as CDs or DVDs, are read with a laser. If you have oil from your skin coating the drive, it will fail to read as the laser will scatter off of the oil. If you just rub the oil off, it’s readable again. [Same thing with scratches](https://cdn.shopify.com/s/files/1/0979/4174/files/disc_cross_sections_73bca5d1-8a54-49d6-94d5-d8389ae2b5f4_large.jpg?13465135418736821126) and why they can be repaired.
In an HDD, the platters are read by a read/write head that hovers as low as 3 nanometers above the disc drive (the first [commercial HDD back in 1956](https://images.computerhistory.org/storageengine/1956_RAMAC_P1.jpg) had a flying height of 5100nm). For reference, a human hair is around 80,000nm wide. Any dust, oils, etc. will be large enough that it’ll either wreck the read/write head or scratch the drive itself, making the platter unreadable. Linus Tech Tips [actually covered this when they toured Drive Savers a while back.](https://www.youtube.com/watch?v=tNUsoangGFs)
The CD data surface is encased in plastic because it’s designed to be handled.
The HDD platters aren’t.
The HDD also has a much higher speed and tighter tolerances because it is safely encased.
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