We've seen for the past several months occasional and random behavior when attempting disk installations after PXE booting to an in-memory version of CoreOS.

What we notice is that the coreos-install will fail and the last error indicates BLKRRPART: Device or resource busy

The work-around we've typically employed it to reboot, PXE again to CoreOS in-memory, then attempt the disk install again, 98-99% of the time we never see the error again and we get a Successful install to disk

Attached is a sample screen of when the random failure happens.

We've seen this on bare-metal blade servers and pizza-box servers, KVM vm's, and Hyper-V vm;s in the past.