re: Building my first NES classic with Raspberry Pi 3

You really don't have to touch the SD card. Once it's formated and retropie is correctly installed on it, you don't have to take it out of the RP3 again. You can transfer ROMs to it remotely over your wifi.

After setting everything up, connecting to your tv and turning on for the first time with the formatted SD card in, the first thing you should do is connect the RP3 to your wifi

Then get on your computer and go to your network settings to find everything connected to your network. You should see "retropie". Click on it and all of the emulator folders will be there. Then DL game ROMs to your computer and copy the file to the appropriate emulator folder there. It'll transfer your ROMs wirelessly over to the SD card that is still plugged into your RP3 that's connected to your tv. When you restart your RP3, your games will now be loaded onto it and play away.

You CAN take out your SD from the RP3 and plug it into your computer and drag the roms to each emulator that's now installed on the SD card. Then plug the SD card back into the RP3 afterwards and restart. But doing it over WiFi is quicker as you don't have to keep taking your SD card in & out to install new games.