MACOS format disallowed

[IronworksProductions]

I used Stephensonsrocket to dualboot macOS and steamos but have since ran into an issue the hard disk is too small and when trying to format the whole hard disk in disk utility recovery mode disk utility fails to unmount the steamos partitions for a full wipe of the hard drive.
Is there something I'm missing that is preventing macOS from erasing my hard drive even in recovery mode with sicp off?

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Thanks,

NN

[sharkwouter]

Try an Ubuntu live cd and use gparted format your disk. The hard drive manager in the Mac OSX installer is really bad.