Parity Check Error Diagnosis

Just noticed my latest parity check identified a large number of errors.  How can I diagnose what's going on and which drive is at fault?

Another thing I noticed, is a slow degradation of the MB/s of the parity check.  Is that caused by more and more data being added to the system?  (I've put 8 additional TB on the system in the past couple years).

Parity/Read-Check History
Date	Duration	Speed	Status	Errors
2017-07-23, 12:58:42	21 hr, 50 min, 31 sec	63.6 MB/s	OK	5066
2017-07-21, 16:48:17	6 hr, 39 min, 41 sec	Unavailable	Canceled	0
2017-07-19, 09:31:54	19 hr, 15 min, 8 sec	72.2 MB/s	OK	0
2017-07-10, 15:46:29	17 hr, 46 min, 28 sec	78.2 MB/s	OK	0
2017-07-03, 18:18:24	20 hr, 18 min, 23 sec	68.4 MB/s	OK	0
2017-06-26, 16:49:28	18 hr, 49 min, 27 sec	73.8 MB/s	OK	3
2017-06-19, 14:53:35	16 hr, 53 min, 34 sec	82.2 MB/s	OK	0
2017-06-15, 02:58:25	14 hr, 7 min, 53 sec	98.3 MB/s	OK	0
2017-05-26, 00:47:10	3 hr, 48 min, 21 sec	Unavailable	Canceled	0
2017-05-15, 18:30:42	19 hr, 30 min, 41 sec	71.2 MB/s	OK	0
2017-05-12, 16:37:33	19 hr, 47 min, 23 sec	70.2 MB/s	OK	0
2017-05-05, 09:42:42	18 hr, 22 min, 7 sec	75.6 MB/s	OK	32
2017-05-01, 09:59:10	10 hr, 59 min, 9 sec	Unavailable	Canceled	0
2017-04-30, 09:13:42	17 hr, 55 min, 35 sec	77.5 MB/s	OK	0
2017-04-24, 17:25:22	18 hr, 25 min, 21 sec	75.4 MB/s	OK	0
2017-04-20, 11:25:34	18 hr, 36 min, 47 sec	74.6 MB/s	OK	0
2017-04-10, 18:27:40	19 hr, 27 min, 39 sec	71.4 MB/s	OK	0
2017-04-03, 19:17:04	20 hr, 17 min, 3 sec	68.5 MB/s	OK	0
2017-03-27, 17:12:47	18 hr, 12 min, 46 sec	76.3 MB/s	OK	0
2017-03-23, 02:22:56	15 hr, 13 min, 5 sec	91.3 MB/s	OK	0
2017-03-20, 17:15:20	18 hr, 15 min, 19 sec	76.1 MB/s	OK	0
2017-03-13, 20:23:27	21 hr, 23 min, 26 sec	64.9 MB/s	OK	0
2017-02-28, 16:05:02	3 hr, 7 min, 48 sec	Unavailable	Canceled	0
2017-02-21, 17:22:48	7 hr, 31 min, 59 sec	Unavailable	Canceled	0
2017-02-20, 10:13:53	11 hr, 13 min, 52 sec	Unavailable	Canceled	0
2017-02-15, 00:02:30	14 hr, 26 min, 21 sec	96.2 MB/s	OK	0
2017-02-13, 16:26:50	17 hr, 26 min, 49 sec	79.6 MB/s	OK	0
2017-02-06, 14:47:19	15 hr, 47 min, 18 sec	88.0 MB/s	OK	
2017-01-30, 14:51:24	15 hr, 51 min, 23 sec	87.6 MB/s	OK	
2017-01-23, 14:49:54	15 hr, 49 min, 53 sec	87.7 MB/s	OK	
2017-01-16, 15:33:54	16 hr, 33 min, 53 sec	83.9 MB/s	OK	
2017-01-09, 14:24:29	15 hr, 24 min, 28 sec	90.2 MB/s	OK	
2017-01-02, 14:16:01	15 hr, 16 min	91.0 MB/s	OK	
2016-12-26, 14:26:04	15 hr, 26 min, 3 sec	90.0 MB/s	OK	
2016-12-24, 12:27:18	5 min, 26 sec	Unavailable	Canceled	
2016-12-19, 14:50:45	15 hr, 50 min, 44 sec	87.7 MB/s	OK	
2016-12-12, 07:29:21	8 hr, 29 min, 20 sec	Unavailable	Canceled	
2016-12-05, 13:58:18	14 hr, 58 min, 17 sec	92.8 MB/s	OK	
2016-12-01, 10:17:17	17 hr, 28 min, 22 sec	79.5 MB/s	OK	
2016-11-28, 12:51:28	13 hr, 51 min, 27 sec	100.2 MB/s	OK	
2016-11-21, 14:07:12	15 hr, 7 min, 11 sec	91.9 MB/s	OK	
2016-11-14, 14:56:02	15 hr, 56 min, 1 sec	87.2 MB/s	OK	
2016-11-11, 19:17:52	46 min, 1 sec	Unavailable	Canceled	
2016-11-08, 18:26:39	4 min, 47 sec	Unavailable	Canceled	
2016-11-07, 13:43:13	14 hr, 43 min, 12 sec	94.4 MB/s	OK	
Oct, 31, 14:07:51	15 hr, 7 min, 49 sec	91.8 MB/s	OK	
Oct, 24, 13:50:21	14 hr, 50 min, 19 sec	93.6 MB/s	OK	
Oct, 17, 13:44:14	14 hr, 44 min, 13 sec	94.3 MB/s	OK	
Oct, 10, 14:04:35	15 hr, 4 min, 33 sec	92.1 MB/s	OK	
Oct, 03, 12:06:59	Unavailable	Unavailable	Canceled	
Sep, 26, 13:58:25	14 hr, 58 min, 24 sec	92.8 MB/s	OK	
Sep, 05, 13:34:45	14 hr, 34 min, 44 sec	95.3 MB/s	OK	
Aug, 29, 14:00:12	15 hr, 10 sec	92.6 MB/s	OK	
Aug, 24, 12:33:59	15 hr, 36 min, 15 sec	89.0 MB/s	OK	
Aug, 22, 12:54:44	13 hr, 54 min, 43 sec	99.9 MB/s	OK	
Aug, 13, 11:09:57	Unavailable	Unavailable	Canceled	
Aug, 08, 13:47:19	14 hr, 47 min, 17 sec	93.9 MB/s	OK	
Aug, 03, 21:02:03	Unavailable	Unavailable	Canceled	
Aug, 01, 14:01:09	15 hr, 1 min, 7 sec	92.5 MB/s	OK	
Jul, 25, 13:48:39	14 hr, 48 min, 38 sec	93.8 MB/s	OK	
Jul, 22, 16:20:10	16 hr, 59 min, 46 sec	81.7 MB/s	OK	
Jul, 18, 12:52:23	13 hr, 52 min, 21 sec	100.1 MB/s	OK	
Jul, 11, 13:40:06	14 hr, 40 min, 4 sec	94.7 MB/s	OK	
Jul, 04, 13:00:08	14 hr, 6 sec	99.2 MB/s	OK	
Jun, 27, 12:56:10	13 hr, 56 min, 8 sec	99.7 MB/s	OK	
Jun, 20, 13:46:52	14 hr, 46 min, 51 sec	94.0 MB/s	OK	
Jun, 13, 12:53:53	13 hr, 53 min, 52 sec	100.0 MB/s	OK	
Jun, 06, 13:15:26	14 hr, 15 min, 24 sec	97.4 MB/s	OK	
May, 30, 13:14:27	14 hr, 14 min, 26 sec	97.5 MB/s	OK	
May, 23, 13:22:05	14 hr, 22 min, 4 sec	96.7 MB/s	OK	
May, 16, 13:06:53	14 hr, 6 min, 51 sec	98.4 MB/s	OK	
May, 09, 13:07:51	14 hr, 7 min, 49 sec	98.3 MB/s	OK	
May, 02, 13:09:50	14 hr, 9 min, 49 sec	98.1 MB/s	OK	
Apr, 25, 13:01:43	14 hr, 1 min, 41 sec	99.0 MB/s	OK	
Apr, 18, 12:59:54	13 hr, 59 min, 53 sec	99.2 MB/s	OK	
Apr, 11, 13:46:56	14 hr, 46 min, 55 sec	94.0 MB/s	OK	
Apr, 04, 12:51:26	13 hr, 51 min, 24 sec	100.3 MB/s	OK	
Mar, 20, 02:50:27	14 hr, 32 min, 27 sec	95.5 MB/s	OK	
Mar, 19, 00:21:18	14 hr, 29 min, 53 sec	95.8 MB/s	OK	
Mar, 14, 20:15:56	14 hr, 11 min, 55 sec	97.8 MB/s	OK	
Mar, 13, 07:10:04	14 hr, 37 min, 34 sec	95.0 MB/s	OK	
Mar, 10, 01:12:11	14 hr, 8 min, 54 sec	98.2 MB/s	OK	
Mar, 08, 08:04:05	14 hr, 9 min, 21 sec	98.1 MB/s	OK	
Mar, 07, 16:15:57	14 hr, 11 min, 43 sec	97.9 MB/s	OK	
Mar, 04, 12:35:00	17 hr, 47 min, 26 sec	78.1 MB/s	OK	
Feb, 29, 12:51:45	13 hr, 51 min, 43 sec	100.2 MB/s	OK	
Feb, 22, 12:51:56	13 hr, 51 min, 55 sec	100.2 MB/s	OK	
Feb, 19, 00:41:40	13 hr, 51 min, 16 sec	100.3 MB/s	OK	
Jan, 25, 19:59:32	20 hr, 59 min, 31 sec	66.2 MB/s	OK	
Jan, 18, 14:12:48	15 hr, 12 min, 46 sec	91.3 MB/s	OK	
Jan, 11, 12:59:07	13 hr, 59 min, 5 sec	99.3 MB/s	OK	
Dec, 28, 18:21:05	19 hr, 21 min, 4 sec	71.8 MB/s	OK	
Dec, 21, 12:51:57	13 hr, 51 min, 56 sec	100.2 MB/s	OK	
Done

Thanks!