

You can set a default OS and a specified length of time to wait for a selection. It would detect all the OS you have in the different partitions and give you a selection menu every time you turn the Pi on for which OS to boot.

install and use a boot manager: For the Raspberry Pi, your best bet for a boot manager is probably BerryBoot.install each OS in a different partition: I don't think NOOBS has that option, this would have to be done as an "advanced user.".Detailed instructions are on the Retropie website: Install Retropie on Raspbian. So yes, you can install Retropie on top of Raspbian. The Retropie images you can download are actually the application on top of Raspbian Lite. Your SD card currently only has 1 partition (because you are using the entire thing), but you could use a partition utility (on your PC or Mac) like fdisk to create more than one. Retropie is a (more or less) normal application. create separate partitions on the SD card: Think of a partition as a "drive".Multiple OS's on the same SD card: my least favoured option. Having everything on the one SD card makes it likely that when the card goes bust, you just may lose everything from both OS's. SD cards are both inexpensive and prone to failure.

Repeat the process you went through to install RetroPi, instead install Raspbian on a new SD card. Separate SD cards: this is always my recommendation.
