The main folders you want to back up are:

/home/pi/RetroPie (ROMs, BIOS files, splash screens) /opt/retropie/configs (configs for RetroArch, EmulationStation, also your scraped game library) /etc/emulationstation(custom themes as well as the systems ES has access to)

The RetroPie documentation has info on how to copy files. WinSCP or Filezilla are probably the easiest programs to use.

Copy those over from the 3B to the 4, and you should be fine. Note: after you flash the image onto your RPi4, you'll need to boot up with it for RetroPie to finish installing, then you can turn it off and start copying.

@LiquidDivide thanks for the awesome scripts!

I made some local changes and hacks to get it working on my own system, not pretty enough to contribute. I hope I get some time to clean it up and contribute it back.

@Clyde Wasn't aware of that tool. Cheers.

Thanks for your reply. I´ll give it a whirl :)

I have now successfully ported to an RPI4.
Some more directories had to be copied.
/etc/emulationstation (All belongs to root)
/home/pi (All owner Pi)
/opt/retropie (Pi is the owner of configs, the rest belongs to root)
Additionally Dosbox had to be updated and the hotkeys had to be redefined.
Furthermore, the HDMI scan line generator had to be supplied externally with 5V (the RPI3 managed the power supply itself).

Also I am downsizing by getting rid of computer emulators
Dos Box,Amiga,and Apple 2

You'll need to use something like pi-shrink to make the image be as large as its contents (vs. the full copy of the card).

https://github.com/Drewsif/PiShrink

@machine_74 this is normal behavior. The only partion that a windows computer can access directly is the boot partition. The other Linux partion, windows doesn't understand what it is and assumes it needs to be formatted. Nothing is wrong with your card, just ignore/close out the format prompts.

@KN4THX Exactly. I wouldn't buy a ssd drive just for a Pi, but when I upgrade the ssd on my main pc, then I could use the older one.

@mitu
The device setup I was thinking would be like this on the raspberry pi
USB1: Usb image boot, raspbian? smaller.
Micro-SDslot: unformatted new microsd (the target of the image write)
USB2: Usb microsd adapter (the source of the image write)

rpi-clone; and I thought I made it sound confusing.

@simpleethat Well, my suggestion is to compress all of your roms into zip or 7z format (as the emulators support) for use on your Pi and to just store them in that format on your NAS. This way they are maximally usable on your Pi should you need to copy them back over.

The amount of extra compression you can get by compressing everything into giant 7z files is fairly small in most cases. There are some roms that can't really be compressed for use by emulators, mostly bin and ISOs. However, most of the emulators also support a compressed format for these types of file (chd, pbp, etc). All the time, effort, and electricity you spend compressing files to eek out a few GB of storage space just isn't worth it.

I’ve used this to copy from a 128g card to a 64g one before to create another Retropie for someone. Worked fine for me apart from having to run it twice just to make sure everything really was copied, but that was just a delta copy then.

https://github.com/billw2/rpi-clone

I plugged the other SD card into a USB adapter and just let it run overnight as I was close to about 60g in actual use.

@Retribution But don't forget that an online backup isn't full under your control and access. It may be that just when you need it, your internet connection doesn't work or the site isn't available.

Also, if you happen to have illegal ROMs, I'd think twice before uploading them to a third-party's server.

Thanks, tried Apple Pie Baker back in the days but thought it was for creating images. Will check it out!