I experienced a similar problem when trying to run the 2020 raspbian images on a recently manufactured Pi 3B+. Black screen, refused to boot, only blinking green ACT light.
This Pi 3B+ was purchased in Nov 2023. Since the 3B+ first retailed in 2018, I was expecting a 2018/2019/2020 raspbian image to work on this Pi 3B+ without a problem. I was wrong. The hardware manufacturers have broken compatibility with these raspbian images by installing new types of SDRAM chips on recently manufactured Pi 3B+ boards. Newer firmware is needed to operate these chips.
So, it is impossible to boot a recently manufactured Pi 3B+ with the firmware from a 2018/2019/2020 raspbian image. All you get is a black screen. The green ACT LED blinks like so:
- 1 blink
- Wait 1 second
- 4 slow blinks
- 8 rapid blinks
- Wait 1 second
- Loop forever
If you are experiencing this problem, you can resolve it by replacing some firmware files from your older raspbian image with their equivalents from a newer raspbian image.
- Replace all
/boot/fixup*.dat files
- Replace all
/boot/start*.elf files
For example, in my case, I was trying to run the 2020-12-02 raspbian release on this recently manufactured Pi 3B+. Only after I replaced the noted firmware files with their equivalents from the 2023-05-03 oldstable release did it finally boot successfully.
Credit to user Dougie for sharing this solution in a comment to this question: Raspberry pi 3b+, 3 green flashes, no display at all
