What happens if one DC goes down in Cassandra multi DC setup

The best way to think of a cassandra database is not as a set of databases on different nodes, but as one single database. Adding the column to the first node adds it to all other nodes. The actual number of times your data is written is determined by your replication strategy and replication factor, but each copy of the data will be the same.

Thus, if you tell the first node about a new column, the second node will automatically understand it and be able to access that data. If you tell the second node about different data in the same column, it will either overwrite the old data or add new data, depending on whether the row you're writing into already has data in it.

If you're adding a new column to the second node, then all the data in that column and the data in the first column will simultaneously exist and can be queried through either/any node.

Q2: If I understand your question correctly, I don't think you want to do that nor should you use Cassandra if that's what you're trying to do. Cassandra is meant to spread data across nodes so that if one node fails then you don't lose your data or crash the entire cluster. If you are putting only specific data one one single hard drive, than you're compromising how Cassandra works.

What's your use case for Cassandra? Why don't you transfer the SSD drive data to a commodity disk then just unplug the SSD and use it for something else? Why do you need Cassandra specifically? Do you have an insane number of writes? What size data are you working with? If you're read heavy you might be better off with a different database.