If you turn RAID off in the BIOS, the rule has historically been that also requires a OS reinstallation. The problem with Windows is it does not like that setting being changed. Windows 10 likely isn't much different.
Because Lenovo was caught with Superfish and LSE, they had to promise not to ship 3rd party bloatware on their laptops to keep their sales. You likely have a Signature Edition system and part of the requirements to be a Signature Edition laptop is Microsoft requires your laptop has to boot up in RAID.
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If you're booting from other installation media, then you'll need to switch from RAID to AHCI temporarily. Once you do what you need, it needs to be switched back to RAID.
If you turn RAID off in the BIOS, the rule has historically been that also requires a OS reinstallation. The problem with Windows is it does not like that setting being changed. Windows 10 likely isn't much different.
Because Lenovo was caught with Superfish and LSE, they had to promise not to ship 3rd party bloatware on their laptops to keep their sales. You likely have a Signature Edition system and part of the requirements to be a Signature Edition laptop is Microsoft requires your laptop has to boot up in RAID.