As reported by Brett Hoerner on solr-user:
It seems that changes in 4.5 collection configuration now require users to
set a maxShardsPerNode (or it defaults to 1).
Maybe this was the case before, but with the new CREATESHARD API it seems a
very restrictive. I've just created a very simple test collection on 3
machines where I set maxShardsPerNode at collection creation time to 1, and
I made 3 shards. Everything is good.
Now I want a 4th shard, it seems impossible to create because the cluster
"knows" I should only have 1 shard per node. Yet my problem doesn't require
more hardware, I just my new shard to exist on one of the existing servers.
So I try again – I create a collection with 3 shards and set
maxShardsPerNode to 1000 (just as a silly test). Everything is good.
Now I add shard4 and it immediately tries to add 1000 replicas of shard4...