i'm able to reproduce the issue when tried from horizon. Since the boot volume gets mounted on 'vda' by default. Future attaches to the VM fail. If the same tried from CLI with the passed (something like : nova boot --flavor 1 --block-device source=volume,id=1730ba2e-5b16-40ae-89d8-745ce6647520,device=/dev/vda,dest=volume,size=2,shutdown=REMOVE,bootindex=0 myvm), volume attaches seem to pass.
i'm able to reproduce the issue when tried from horizon. Since the boot volume gets mounted on 'vda' by default. Future attaches to the VM fail. If the same tried from CLI with the passed (something like : nova boot --flavor 1 --block-device source= volume, id=1730ba2e- 5b16-40ae- 89d8-745ce66475 20,device= /dev/vda, dest=volume, size=2, shutdown= REMOVE, bootindex= 0 myvm), volume attaches seem to pass.