gistfile1.txt

Assuming that each 
+-------+
|Primary|
|  75GB |
+-------+
is a separate server

Single MongoDB Server
If this server dies, all data is unavailable
+-------+ 
|Primary|
|  75GB |
+-------+ 


MongoDB with Replica Sets
If the primary dies, the secondary will take over for the entire database

     Replica Set              
         |
         |
        / \
+-------+ +---------+ 
|Primary| |Secondary|
|  75GB |=| 75GB    | 
+-------+ +---------+ 

MongoDB with Sharding
If a server dies, then all the data inside the shard that died will become unavailable.

                            Sharded Cluster             
             /                    |                    \
      Shard A                  Shard B                  Shard C
         |                        |                        |
     +-------+                +-------+                +-------+ 
     |Primary|                |Primary|                |Primary|
     |  25GB |                | 25 GB |                | 25 GB |
     +-------+                +-------+                +-------+

MongoDB with Sharding and Replication 
If a server dies, then if it was a primary for a shard, then only that shard will move over to its secondary

                            Sharded Cluster             
             /                    |                    \
      Shard A                  Shard B                  Shard C
        / \                      / \                      / \
+-------+ +---------+    +-------+ +---------+    +-------+ +---------+
|Primary| |Secondary|    |Primary| |Secondary|    |Primary| |Secondary|
|  25GB |=| 25GB    |    | 25 GB |=| 25 GB   |    | 25GB  |=| 25GB    |   
+-------+ +---------+    +-------+ +---------+    +-------+ +---------+

RethinkDB (replica=1, shard=1)
If this server dies, all data is unavailable
+-------+ 
|Primary|
|  75GB |
+-------+ 

RethinkDB (replica=1, shard=3)
If a server dies, then all the data inside the shard that died will become unavailable.
                            RethinkDB Cluster             
             /                    |                    \
      Shard A                  Shard B                  Shard C
         |                        |                        |
     +-------+                +-------+                +-------+ 
     |Primary|                |Primary|                |Primary|
     |  25GB |                | 25 GB |                | 25 GB |
     +-------+                +-------+                +-------+

RethinkDB (replica=3, shard=1)
If a server dies, one of the secondaries will become the new primary

   RethinkDB Cluster              
               |
               |
        /      |        \
+-------+ +---------+ +---------+ 
|Primary| |Secondary| |Secondary|
|  75GB |=| 75GB    |=| 75GB    |  
+-------+ +---------+ +---------+ 


RethinkDB (replica=3, shard=3)
If a server dies, then the affected shard will elect a new primary replica
                                          RethinkDB Cluster
                /                              |                              \
             Shard A                         Shard B                        Shard C
        /      |        \               /      |        \               /      |        \       
+-------+ +---------+ +---------+ +-------+ +---------+ +---------+ +-------+ +---------+ +---------+
|Primary| |Secondary| |Secondary| |Primary| |Secondary| |Secondary| |Primary| |Secondary| |Secondary|
|  25GB |=| 25GB    |=| 25GB    | |  25GB |=| 25GB    |=| 25GB    | |  25GB |=| 25GB    |=| 25GB    |
+-------+ +---------+ +---------+ +-------+ +---------+ +---------+ +-------+ +---------+ +---------+