ReadyNAS 4220 - Disk expansion Volume 'data' is 'DEGRADED'.

Missing some output there. What you have pasted is the partitions active in the swap array (md1) which we're not too concerned about in this case. We'll want to see the status of the data array, should be towards the top of the mdstat.log file and look something like this:

Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md127 : active raid5 sdi3[0] sdf3[5] sde3[4] sdb3[3] sda3[2] sdj3[1]
19510844160 blocks super 1.2 level 5, 64k chunk, algorithm 2 [6/6] [UUUUUU]

md1 : active raid6 sda2[0] sdm2[11] sdl2[10] sdj2[9] sdi2[8] sdh2[7] sdg2[6] sdf2[5] sde2[4] sdd2[3] sdc2[2] sdb2[1]
5237120 blocks super 1.2 level 6, 64k chunk, algorithm 2 [12/12] [UUUUUUUUUUUU]

md0 : active raid1 sda1[0] sdm1[11] sdl1[10] sdj1[9] sdi1[8] sdh1[7] sdg1[6] sdf1[5] sde1[4] sdd1[3] sdc1[2] sdb1[1]
4190208 blocks super 1.2 [12/12] [UUUUUUUUUUUU]