I have to say I formatted it as FAT32 yet it failed to be formatted..
diskutil wasn't able to unmount the volume before formatting it.
I suspect the reasons being i) the filesystem or partition map have been damaged ii) there's some physical damage to the card
The card couldn't even be read or recognized by the machine unless I put "diskutil list" command (equivalent to "fdisk -l" in Linux) to list all the drives connected to my system.
But now that I've got a working card, I guess I'll just throw that card away.