Booting Back Into OS X From Bootcamp

[adyf]

I bought a GDrive external hard drive with thunderbolt connectivity this week so I can start using Time Machine. I've had it connected for the last few days with no issue but today I booted into bootcamp to use Windows and when I had finished I right clicked on the bootcamp icon in the task bar and chose the option to 'restart in OS X'. What happened next surprised me, it booted into Recovery Mode (OS X Utilities). It doesn't do this if I disconnect the GDrive or I eject the GDrive from the task bar before restarting in OS X, therefore I suspect all is well in terms of hard drive/external drive wellbeing etc. Can anyone explain this or has had a similar thing happen to them? Thanks in advance.