unable to mkfs.btrfs ERROR: unable to open /dev/sdc: Device or resource busy

Solution 1:

A possibility, that happened to me, is that the hard drive was at some point a component of a software RAID. In such case, even if "deleting" its partition table, and rebooting, there will be some persistent data in the superblock that causes the OS (CentOS 7 in my case) to recognize that and to create an entry such as /dev/md127 in the /dev directory.

Now that I want disk this just to work stand-alone, no more RAID association to it, you'll need to tell the system, such as:

# mdadm --stop /dev/md127

(You may use mdadm --detail /dev/md127 to see metadata beforehand).

After the stop, that metadata is gone, and the disk can be reused.