User beware… If you upgrade your VirtualBox too far ahead of the guest additions installed in Vagrant then you can end up with an instability that will trash your disks in the vagrant box and cause the file system to go read only.

I kept having readonly filesystem problems and they were getting more and more frequent. After combing the vagrant and virtualbox forums I saw some hints that it might be related to Guest Additions being too far out of date with the main VirtualBox version. After following the instructions here to get my guest additions up to date I’ve had no further trouble with a readonly file system.