High disk watermark [90%] exceeded on node,shards will be relocated away from this node

Yes, i think. I haven't run into this issue though.

However, you may consider other other options as well - if you have indices, rolling daily, you may delete old indices.

You may refer to retiring data section in the definitive guide and snapshot and restore links below.

https://www.elastic.co/guide/en/elasticsearch/guide/current/retiring-data.html
https://www.elastic.co/guide/en/elasticsearch/reference/2.4/modules-snapshots.html