Hi Robert,
Thank you for your prompt reply.
I tried several SD cards. It seems they are not worn or dirty pins. If those cards plugged into BBBw SD card reader on BBBw dev boards, they can be detected as /dev/mmcblk0 and /dev/mmcblkp1. lsblk command also lists those devices.
But If I plugged them into Ubuntu PC, dmesg shows many errors. ls, lsblk, lsusb commands show no devices.
If I plug them using USB sd card reader (plugged into usb port) into Ubuntu PC, it is listed as /dev/sdb. If fdisk /dev/sdb, it returns fdisk: cannot access '/dev/sdb: no media found.
Both usb sd card reader and sd reader are ok if a normal sd card is plugged in.
Best Regards,
David