To say it in a MS Windows terms (which also doesn’t support this). Lets say you have server A and you share a directory. On server B you connect Server A’s share to the W: drive. On server B you are try to share the W: drive. The share fails because you are trying to reshare a mounted share. Again this is a file level share. You may be able to do this with a block level share (as an iSCSI volume), but I haven’t tried.