Creating software raid using mdadm fails with "Device or resource busy"

Issue

• Creation of a software RAID 1 didn't succeed because devices are marked as busy even if fresh attached to the system.

# mdadm -C --verbose /dev/md3 -l1 -n2 /dev/sds1 /dev/sdt1 
mdadm: Cannot ope1n /dev/sds1: Device or resource busy
mdadm: Cannot open /dev/sdt1: Device or resource busy
mdadm: create aborted.

• Here are the disk details:

# fdisk
[...]
Disk /dev/sds: 16.2 GB, 16220160000 bytes
64 heads, 32 sectors/track, 15468 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sds1               1       15468    15839216   fd  Linux raid autodetect

Disk /dev/sdt: 16.2 GB, 16220160000 bytes
64 heads, 32 sectors/track, 15468 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdt1               1       15468    15839216   fd  Linux raid autodetect