The user manual for your motherboard illustrates the placement of the SATA connectors.
On a
MAXIMUS VIII EXTREME, "SATA6G_1" will always be the default (AHCI Mode) boot drive. Whichever SATA drive is plugged into it should be the boot/system drive, unless you've configured a multi-drive SATA RAID or you're using a PCIe/M.2 SSD.
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