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!

@mppsan If the system you're copying to has also RetroPie, chances are the system is loading you USB pendrive in the ROMs folder and you're just re-copying the files on the usb. Disable the USB mount service from the RetroPie-Setup script and then mount the USB pendrive manually from the command line.

@mitu Thank you! I can't believe I didn't stumble upon those in my search.

@mitu Thanks a lot.

This saves me a lot of time, preparing an rsync just for those folders and ignoring entirely /usr, /bin, etc.

I also noticed the existence of /home/pi/.emulationstation that includes stuff like the "scraped" game images, but I found it to be a link to /opt/retropie/configs/all/emulationstation .

I also noticed that the .state files are stored inside the "roms" folders, in the same subfolder than the rom itself.

Thanks and best regards.

@mitu Okay ,thanks for your help :)