Originally Posted by hwoarang
I need to do so because my computer is booting on a SD card and I only have 1 SD card slot. Before making a mistake, I would like to make a backup of the SD card. With dd, I was able to make an image of the sd card and I would like to copy it to another one. But as a partition from the SD card is mounted as /, I cannot unmount it to swap the sdcard...
Thanks for your attention.
Then you need an alternate boot method. You definitely can't umount your root file system.
Can't you boot from a livecd, an hd or any other thing?
There was a project that aimed at the possibility to boot a new kernel from userspace (which seemed pointless to me because it only saves you the 5 seconds that the POST test of the BIOS takes if you reset). Before you ask: no, I have no idea of it's name anymore and I have no idea of the state of that project either.
Other than booting from another device I see no easy way around your problem. If you can't do that, just take the drives to a pc with two card hubs and clone it. It will be easier. Alternatively, buy a card reader/writer and attach it to an usb port. I see drives of this kind on some shops nowadays around 6-10€ which accept all kind of sd card formats.