root # mknod /dev/md1 b 9 1
root # mknod /dev/md2 b 9 3
root # mknod /dev/md3 b 9 4

root # ls -al /dev/md*
brw-r--r-- 1 root root 9, 1 Dec  2 02:08 /dev/md1
brw-r--r-- 1 root root 9, 3 Dec  2 02:14 /dev/md2
brw-r--r-- 1 root root 9, 4 Dec  2 02:14 /dev/md3

root # mdadm -A /dev/md1 /dev/hda1 /dev/hdb1
mdadm: /dev/md1 has been started with 2 drives.

root # ls -al /dev/md*
lrwxrwxrwx 1 root root    4 Dec  2 02:15 /dev/md1 -> md/1
brw-r--r-- 1 root root 9, 3 Dec  2 02:14 /dev/md2
brw-r--r-- 1 root root 9, 4 Dec  2 02:14 /dev/md3

/dev/md:
total 0
drwxr-xr-x  2 root root   60 Dec  2 02:15 .
drwxr-xr-x 15 root root 3340 Dec  2 02:15 ..
brw-rw----  1 root disk 9, 1 Dec  2 02:15 1

root # mdadm -A /dev/md2 /dev/hda3 /dev/hdb3
mdadm: /dev/md2 has been started with 2 drives.

root # ls -al /dev/md*
lrwxrwxrwx 1 root root    4 Dec  2 02:15 /dev/md1 -> md/1
brw-r--r-- 1 root root 9, 3 Dec  2 02:14 /dev/md2
lrwxrwxrwx 1 root root    4 Dec  2 02:16 /dev/md3 -> md/3

/dev/md:
total 0
drwxr-xr-x  2 root root   80 Dec  2 02:16 .
drwxr-xr-x 15 root root 3340 Dec  2 02:16 ..
brw-rw----  1 root disk 9, 1 Dec  2 02:15 1
brw-rw----  1 root disk 9, 3 Dec  2 02:16 3

root # mdadm -A /dev/md2 /dev/hda3 /dev/hdb3
mdadm: device /dev/md3 already active - cannot assemble it