The RetroPie RPI image is just a Raspi OS Lite installation, so as far as booting is the same as a Raspi OS image. Just write the image to the USB disc and boot from it. With any recent Pi4 revision, it should boot from USB automatically.
@kronos Yes. I don't know if it's completely out of beta for the Pi 4 yet, but the procedure is decribed here. And you'll need some sort of USB->SATA converter of course or simply an external USB HDD/SSD.
OK, I got this working. To boot from SSD using the RetroFlag NESPI 4 case, I needed two things. First, I needed to install RetroPie to an 8 GB micro SD card. Then, I needed to run that linux from the SD card to apply the tweaks described in this file here: http://download.retroflag.com/Products/NESPi_4_CASE/How_to_boot_from_SSD.zip Then, I needed to make an image of the SD card on my hard drive, and then write that image to the SSD. Then I could finally put in the SSD and have it boot.
Unfortunately, this means disabling UAS, which greatly reduces the speed of the SSD. It doesn't completely defeat the purpose because it's still faster than the SD card, but not by as much as it should be.This is the first time I got a bad product from RetroFlag. Their GPI case was overpriced but works great. RetroFlag's NESPI 3B+ case was amazing, the best gaming case you could get for the Raspberry PI 3 B+, but I would strongly recommend against purchasing the RetroFlag NESPI 4 case if you plan to use the SSD option at all, because it doesn't work with UAS so you don't get the full speed of the SSD.
I should have just stuck with the Raspberry PI 3B+ and skipped the 4 generation, waiting for the Raspberry PI 5 or 6 or whatever, which should hopefully have proper SSD support and be able to emulate the Gamecube and PS2, cure cancer and solve world hunger someday
@george-spiggott it depends what we're comparing, but realistically I wouldn't see why there would be much of an improvement between the 2 on the Pi performance wise, because the architecture just isn't there to take full advantage of the SSD's max speeds unless you use USB 3.0 external ones.
also high endurance SD cards exist which were intended for dashcam and security camera systems, life expectancy on a Raspberry Pi using these SD cards shouldn't be a problem for many years if all you're doing is emulating old games on them which will result in fewer rewrites per day.
The only big tangible improvement that would come from using an SSD is capacity. Geekworm sells a case that lets you install an internal one using expansion boards, there isn't anything wrong with trying it, but it's probably a waste of time if all you're using it for is emulating retro games.
I am guessing retropie still will write heavily to the sd card despite roms being on an external, correct? Not sure if sd wear is a big deal with retropie, so figured I'd ask if it was and if there's a similar way to mitigate.
Not really, gaming is not a write-heavy process. Game saves and save states are written to the ROM's folder by default and unpacking .zip ROMs is usually done in RAM.
There is no constant write load using RetroPie, like a monitoring/data collection/aggregation process entails, so a good sdcard and proper power source is usually enough.