I moved mongo files (*.ns, *.0, *.1, etc.) to an external drive, then upgraded to the latest mongo version, After I couldn't access those files anymore, but this sort of thing had worked with earlier versions.
How do I reverted to a previous version of mongo (MMAPv1), that would allow this, or access this data from the current version? I think WiredTiger was the problem.
Thank you!
MongoDB 3.4 contains the original MMAPv1 storage engine.
If you want to read MMAP data files created by an another version of MongoDB you can. Start up a new instance of mongod with the following snippet.
mongod --storageEngine mmapv1