For a lot of reasons, it may be convenient to detach or attach live disks to a running VM without having to reboot it.  Sure, you can use some network based storage, but when performance counts, attaching a new virtual disk will usually give you better throughput and lower latency in a quick testing situation.

This doesn't work, why not?

drive_add 0 if=virtio,file=/tmp/vm.qcow2,if=virtio,format=qcow2,id=rtt

Can't hot-add drive to type 7

You need to do add the drive but without attaching it instead:

We achieve this by setting if=none so it has no physical interface but we make QEMU aware that the virtual disk does exist.

drive_add 0 if=virtio,file=/tmp/vm.qcow2,if=none,format=qcow2,id=rtt

Now we can hot add the drive to the OS by referencing the id which we defined as rtt:

device_add virtio-blk-pci,drive=rtt

There should be no output which means it is all good now!

Congrats, you've now hotadded a virtual drive to QEMU without having to restart

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