Btrfs devel - Nov 2014 - scrub implies failing drive - smartctl blissfully unaware

Hey, guys

See further below extracted output from a daily scrub showing csum 
errors on sdb, part of a raid1 btrfs. Looking back, it has been getting 
errors like this for a few days now.

The disk is patently unreliable but smartctl's output implies there are 
no issues. Is this somehow standard faire for S.M.A.R.T. output?

Here are (I think) the important bits of the smartctl output for 
$(smartctl -a /dev/sdb) (the full results are attached):
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE UPDATED  
WHEN_FAILED RAW_VALUE
   1 Raw_Read_Error_Rate     0x000f   100   253   006    Pre-fail 
Always       -       0
   5 Reallocated_Sector_Ct   0x0033   100   100   036    Pre-fail 
Always       -       1
   7 Seek_Error_Rate         0x000f   086   060   030    Pre-fail 
Always       -       440801014
197 Current_Pending_Sector  0x0012   100   100   000    Old_age 
Always       -       0
198 Offline_Uncorrectable   0x0010   100   100   000    Old_age 
Offline      -       0
199 UDMA_CRC_Error_Count    0x003e   200   200   000    Old_age 
Always       -       0
200 Multi_Zone_Error_Rate   0x0000   100   253   000    Old_age 
Offline      -       0
202 Data_Address_Mark_Errs  0x0032   100   253   000    Old_age 
Always       -       0



-------- Original Message --------
Subject: 	Cron <root@watricky> /usr/local/sbin/btrfs-scrub-all
Date: 	Tue, 18 Nov 2014 04:19:12 +0200
From: 	(Cron Daemon) <root@watricky>
To: 	brendan@watricky



WARNING: errors detected during scrubbing, corrected.
[snip]
scrub device /dev/sdb2 (id 2) done
	scrub started at Tue Nov 18 03:22:58 2014 and finished after 2682 seconds
	total bytes scrubbed: 189.49GiB with 5420 errors
	error details: read=5 csum=5415
	corrected errors: 5420, uncorrectable errors: 0, unverified errors: 164
[snip]