Can I use my old MyCloud (OS3) as a backup drive for new MyCloud Home?

The My Cloud Home is OS 4 and is not OS 5 compatible. As of this writing, the OS 4 is still a hybrid OS using Android API (although this may change with any up coming firmware) for on device apps and MCH REST API and OAuth for off device applications. You can use the USB of the MCH for backup to USB HDD and that is still the fastest way to copy data.