Mirrored Raid 1 showing up as two drives in disk management

So its really weird the machine was set to AHCI and it was seeing the old raid array just fine in windows. It did have 2 hdds in a mirror. It WAS IN AHCI yet the mirror held. I wanted to update to ssd. SO. I put the bios in raid and made the array (on 2 ssd drives) and I restored the two hdds to the ssds in with a image. Because the old array was running just fine in AHCI mode ,when windows 10 booted up it encountered and error and would not continue. I assumed it was because when the old drives were working in AHCI mode that the bios needed to back to that. When i set the bios to AHCI windows booted just fine. It seams to me a mirrored pair can be a liability in older hardware. Windows 10 is not supported on this machine anymore its a older intel dual core. I chose to pull one of the ssd s out of the mirror and make it a normal setup. It would be different if the machine was still supported by intel and windows. Id rather have an extra drive that I can use to put my desktop and data on so its easer to restore the os and not worry about not getting at my data because my os took a dump. These older machines have usb 2 so that complicates backing up to an external drive.
It is a mystery to me WHY the machine was in AHCI mode. ANY ONE WITH LINK CAN VIEW
This is a link to the manual and the raid setup guide. If anyone is interested. I find it really weird that this machine was in AHCI before I did anything to it yet it showed the raid as a ONE virtual drive in disk management.