Tips and Tricks for Troubleshooting
Troubleshooting (device does not boot or alike)
check SD card format
check if the SD card is formatted correctly. It needs to have two partitions, one FAT and one ext2/3. The FAT partition needs to have a specific setup of number of sectors, cylinders etc. that is different from the defaults! And, the file MLO must be copied as the first file to the freshly formatted partition. Even copying some file and deleting them before MLO is copied makes the SD card unbootable.
check console output
Connect the RS232 cable and start some terminal program.
You should see a "message" like "40W" (OMAP3530) or "60" (DM3730). This indicates that the Boot ROM is starting and using the RS232 interface.
Right after this you will see a boot message from X-Loader and then from U-Boot and finally the Linux kernel loading. If you see errors there, they are hints what is going wrong.
how to Boot and unbrick through RS232 if neither NAND nor SD card works
see a complete description at SerialBoot
debug using the serial console
To log in either into uboot or in a serial terminal first build the serial cable as described in the Manual. Afterwards, connect the serial cable through a serial to usb converter to your pc. A corresponding device at /dev/ttyUSB0 should appear (consult dmesg). Use a terminal emulation programm such as minicom or agetty to log in using this device (e.g. minicom -D /dev/ttyUSB0). Remember to deactivate the hardware control flow option, otherwise you won't be able to type inside the serial console.