As I manufacture ArcaOS systems, the computers I start with often already have Windows 10 installed. Starting with ArcaOS version 5.0.4, the OS2AHCI driver will not allow access to GPT formatted hard drives. Fortunately for me, this new behavior can be defeated.
The ArcaOS installer may not see the disk at all with default settings. This is a feature to protect Windows from being overwritten. Windows is normally installed to a GPT format hard drive, while ArcaOS is installed to a MBR format hard disk. A hard drive can only be one format, so in order to install ArcaOS, I need to wipe the hard drive.
In order to wipe the hard drive, I need to boot with a menu to change from the default settings. I page through the menu until I see the OS2AHCI disk driver, and I add the following parameter:
BASEDEV=OS2AHCI.ADD /!U
After booting with this setting, ArcaOS Disk Utility can find the drive, and I can wipe. Once wiping had been done, the computer will see the hard drive even with default settings. OS/2 and therefore ArcaOS, is very picky about drive geometry. A disk laid out by any other operating system will likely not work under OS/2. I always use OS/2-based tools to partition hard drives.
I wrote the above before ArcaOS 5.1 added GPT disk support. Now, ArcaOS can be installed to a GPT partitioned disk without erasing the disk first.