command to format an sd card

Hello Guys,
I'm trying to format a sd card.
I'm using a embedded linux system.
i sent this command.
First Test:
> /bin/umount -l /dev/mmcblk0p1 --> unmount the Sd card.
> /sbin/mkdosfs -I /dev/mmcblk0p1

my device on this command answer :

mkfs.fat 3.0.26 (2014-03-07)
mkdosfs: unable to open /dev/mmcblk0p1: Device or resource busy

Second test
> bin/umount -l /dev/mmcblk0p1 --> unmount the Sd card.
> mkfs.fat -F 32 /dev/mmcblk0p1
Device answer:

mkfs.fat 3.0.26 (2014-03-07)
mkfs.fat: unable to open /dev/mmcblk0p1: Device or resource busy



What can i do?
how can i format my SD card from my system?

Welcome to LinuxQuestions.

What SBC are you running?

I assume your embedded system boots and runs from the SD card. If /dev/mmcblk0p1 is your /root partition it can not be unmounted and is why you see the "Device or resource busy" error message.

What are you trying to accomplish?

was there any respond to the umount command? Did you check the return code of it?

yes, make sure it is NOT mounted, even if you have to pop it out the push it back in to engage it again, not letting a file manger mount it. also make sure that the tab is slid to write protection off.

I agree it will help to know what type of embedded hardware you are using and what operating system.

But the output I would most like to see is if you enter the "lsblk" command. It will show all block devices, along with their partitions, and whether they are mounted or not.

I am also not sure it is helpful to run "umount -l" instead of just plain "umount". Without the "-l", "umount" will tell you it cannot unmount because the partition on the card is being used.