Hmm. that seems very strange. But if you are sure that the MD is gone from the Devices Tree (check!), then you will also see that its image is gone from /usr/pluto/diskless/#/ where # is the device number of your MD as it was before you deleted it. In other words, this directory should now be empty. If so, then all you have to do is reboot your MD and let it PXE boot from the core once again.... this will setup another MD for you as you did before, no other preparation is needed, just boot the MD. None of this has anything to do with the core/hybrid, you do not need to rebuild that.