Adding a drive later is not a function of InstantCake (unless you are using InstantCake to rebuild as a dual drive system). If you manually added the second drive yourself, its a function of the disk you used for booting and runnign mfsadd; it sounds like the one you used does not have an LBA48 kernel.
For answers to commonly asked questions, please refer to the
InstantCake Instructions and Release Notes,
Official InstantCake Discussion Thread, and the
Official PTVnet Discussion Thread
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