When SlaveOk is specified in the connection string, the DropCollection command fails as the driver uses the generic FindOneAs() function to send the command to mongodb. FindOneAs() will pick a slave as it thinks it is a read operation and SlaveOk has been set.

Steps to reproduce:

1) Create a mongodb replica set with at least one master and one slave

2) Use a connection string that specifies SlaveOk

3) Execute DropCollection on a database

4) "Not master" exception is thrown

Callstack (line numbers may be incorrect):
> MongoDB.Driver.dll!MongoDB.Driver.MongoDatabase.RunCommandAs<MongoDB.Bson.BsonDocument,MongoDB.Driver.CommandResult>(MongoDB.Bson.BsonDocument command) Line 343 C#
MongoDB.Driver.dll!MongoDB.Driver.MongoDatabase.RunCommand<MongoDB.Bson.BsonDocument>(MongoDB.Bson.BsonDocument command) Line 319 + 0x30 bytes C#
MongoDB.Driver.dll!MongoDB.Driver.MongoDatabase.DropCollection(string collectionName) Line 178 + 0x10 bytes C#

Root cause:
var result = CommandCollection.FindOneAs<TCommand, TCommandResult>(command); // this will cause a slave to be chosen as the connection