Last week I flashed Raspbian Buster image to a very old SanDisk 8gb card (slow) using win32diskimager on windows 10, it worked just fine on my new RPI4B-1GB board - and its still functional ...
Steps followed : 1) format SD card on windows as a FAT32 volume 2) flash 2019-07-10-raspbian-buster.img via win32diskimager
This 8gb card boots just fine in the new RPI-4B and older RPI-3B+.
I am now following the same procedure with the exact same Buster image (week-old and downloaded fresh) on 3 Sandisk 16GB ultra SDHC 1 cards for more disk space and none of them boot either in the new RPI-4B nor the older RPI-3B+ boards.
On the old RPI-3B+ - just the red power led stays on - no activity on green led - no hdmi output. On the new RPI-4B - as the red power led stays on the green led starts flashing 4 times periodically - no hdmi output.
All the 4 cards had old RPI3 images ( they mounted 2 partitions - boot and root when connected to the usb ports on windows 10 laptop). While the first format+flashing of 8gb card was successful right away, the other 16gb have been FAT32-formatted and buster-image-flashed multiple times.
Also 1) The cards are not corrupt. 2) I can format all of them to NTFS and access their full capacity that way 3) they all had bootable ubuntu-mate RPI 3B+ images on them earlier.
while I strongly suspect its a boot-sector corruption/alignment issue, I am totally stumped on this inexplicable issue...
what could be causing the problem here ? what else can I try?