Software RAID replace failed harddisk

cat /proc/mdstat Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] md3 : active raid1 sda4[3] sdb4[2] 2778744127 blocks super 1.2 [2/1] [_U] [>………………..] recovery = 1.3% (38884416/2778744127) finish=534.3min speed=85452K/sec md2 : active raid1 sda3[3] sdb3[2] 134216632 blocks super 1.2 [2/2] [UU] md1 : active raid1 sda2[3] sdb2[2] 524276 blocks super 1.2 [2/2] [UU] md0 : […]