title: mongo upgrade and now my data is incompatable
body: <p>I did a brew update/upgrade today on my mac and then mongodb caused my mac to crash badly.  I'm trying now to get my data into an Atlas cluster. All I have now are my /data/db files.  I need to dump them to port them to Atlas.  Rather than mess up my mac I moved to Ubuntu.  On Ubuntu I installed mongo 3.6.6.  I tried to start <code>mongod</code> and got the messages below.  I googled and googled and tried tons of things without success.  Please advise.</p>
  
  <pre><code>root@risksheet:/opt/marc# mongod
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten] MongoDB starting : pid=26111 port=27017 dbpath=/data/db 64-bit host=risksheet
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten] db version v3.6.6
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten] git version: 6405d65b1d6432e138b44c13085d0c2fe235d6bd
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten] OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten] allocator: tcmalloc
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten] modules: none
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten] build environment:
  2018-08-09T21:35:01.592-0400 I CONTROL  [initandlisten]     distmod: ubuntu1604
  2018-08-09T21:35:01.593-0400 I CONTROL  [initandlisten]     distarch: x86_64
  2018-08-09T21:35:01.593-0400 I CONTROL  [initandlisten]     target_arch: x86_64
  2018-08-09T21:35:01.593-0400 I CONTROL  [initandlisten] options: {}
  2018-08-09T21:35:01.593-0400 I -        [initandlisten] Detected data files in /data/db created by the 'wiredTiger' storage engine, so setting the active storage engine to 'wiredTiger'.
  2018-08-09T21:35:01.593-0400 I STORAGE  [initandlisten] 
  2018-08-09T21:35:01.593-0400 I STORAGE  [initandlisten] ** WARNING: Using the XFS filesystem is strongly recommended with the WiredTiger storage engine
  2018-08-09T21:35:01.593-0400 I STORAGE  [initandlisten] **          See http://dochub.mongodb.org/core/prodnotes-filesystem
  2018-08-09T21:35:01.593-0400 I STORAGE  [initandlisten] wiredtiger_open config: create,cache_size=991M,session_max=20000,eviction=(threads_min=4,threads_max=4),config_base=false,statistics=(fast),cache_cursors=false,compatibility=(release="3.0",require_max="3.0"),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),statistics_log=(wait=0),verbose=(recovery_progress),
  2018-08-09T21:35:02.492-0400 E STORAGE  [initandlisten] WiredTiger error (-31802) [1533864902:492320][26111:0x7f9ce698c9c0], connection: Version incompatibility detected: unsupported WiredTiger file version: this build requires a maximum version of 2, and the file is version 3: WT_ERROR: non-specific WiredTiger error
  2018-08-09T21:35:02.498-0400 E -        [initandlisten] Assertion: 28595:-31802: WT_ERROR: non-specific WiredTiger error src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp 421
  2018-08-09T21:35:02.498-0400 I STORAGE  [initandlisten] exception in initAndListen: Location28595: -31802: WT_ERROR: non-specific WiredTiger error, terminating
  2018-08-09T21:35:02.498-0400 I NETWORK  [initandlisten] shutdown: going to close listening sockets...
  2018-08-09T21:35:02.498-0400 I NETWORK  [initandlisten] removing socket file: /tmp/mongodb-27017.sock
  2018-08-09T21:35:02.498-0400 I CONTROL  [initandlisten] now exiting
  2018-08-09T21:35:02.498-0400 I CONTROL  [initandlisten] shutting down with code:100
  root@risksheet:/opt/marc# 
  </code></pre>
  
  <p>I also tried setting the engine to <code>WiredTiger</code> but that didn't help.</p>