Setup: There's 3x power supplies powering the system. One to mainboard, one to normal HDD's and Fans, and one to RAID array. The RAID is a 7 stack RAID5 using SATA interface for a SAS controller (hardware, not onboard). There's 5 SATA drives connected directly to the mainboard with the onboard RAID disabled, and the RAID card set as first boot device in sequence. The RAID is setup as a single chunk with an MBR partitioning on it contaning 3 primary partitions, ntfs, ntfs, ext4. GRUB was selected to install into the MBR during instalation. It is GRUB2.
Problem: When booting up to grub with everything on, grub will dumb to rescue mode reporting inability to find by UUID. Uppon performing the command ls, only the SATA drives show up in the list and grub is not seeing the RAID array it is installed onto and booting from. It the SATA drives are disconnected (or simply turned off) during boot, GRUB will boot correctly, showing proper menu entries, at which stage I am able to turn the SATA drives back on and things will go as normal with proper mounting of all partitions/drives.
Key notes: My current OS is highly irrelevant here as this is reproducible with any OS installed, this is an issue with GRUB and not the OS.
TLDR version: GRUB2 doesn't see the hardware RAID it's installed onto if other drives are connected to onboard SATA interface.