My latest parity check says there are 128 disk errors on one disk. What to do next?

Go to support? RTFM? :-)

Tools -> Diagnostics

 

Post the .zip file

Here it is - thanks in advance...

nas2-diagnostics-20170606-0810.zip

2 hours ago, benflux said:

Here it is - thanks in advance...

nas2-diagnostics-20170606-0810.zip

 

I am not sure. Your log is full of these. You need someone more familiar with syslogs to decipher.

 

Jun  6 04:41:01 NAS2 shfs/user: err: shfs_chown: lchown: /mnt/cache/Android 99 100 (1) Operation not permitted
Jun  6 04:41:01 NAS2 emhttp: err: shcmd: shcmd (53617): exit status: 1
Jun  6 04:41:01 NAS2 emhttp: shcmd (53618): chmod 0777 '/mnt/user/EFI'
Jun  6 04:41:01 NAS2 emhttp: shcmd (53619): chown 'nobody':'users' '/mnt/user/EFI'
Jun  6 04:41:01 NAS2 shfs/user: err: shfs_chown: lchown: /mnt/cache/EFI 99 100 (1) Operation not permitted
Jun  6 04:41:01 NAS2 emhttp: err: shcmd: shcmd (53619): exit status: 1
Jun  6 04:41:01 NAS2 emhttp: shcmd (53620): chmod 0777 '/mnt/user/domains'
Jun  6 04:41:01 NAS2 emhttp: shcmd (53621): chown 'nobody':'users' '/mnt/user/domains'
Jun  6 04:41:01 NAS2 shfs/user: err: shfs_chown: lchown: /mnt/cache/domains 99 100 (1) Operation not permitted
Jun  6 04:41:01 NAS2 emhttp: err: shcmd: shcmd (53621): exit status: 1
Jun  6 04:41:01 NAS2 emhttp: shcmd (53622): chmod 0777 '/mnt/user/system'
Jun  6 04:41:01 NAS2 emhttp: shcmd (53623): chown 'nobody':'users' '/mnt/user/system'
Jun  6 04:41:01 NAS2 shfs/user: err: shfs_chown: lchown: /mnt/cache/system 99 100 (1) Operation not permitted
Jun  6 04:41:01 NAS2 emhttp: err: shcmd: shcmd (53623): exit status: 1

I looked at your drives, however, and the one with serial number ending in X8Z has some issues: 

 

X8Z

ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
183 Runtime_Bad_Block       0x0032   088   088   000    Old_age   Always       -       12
199 UDMA_CRC_Error_Count    0x003e   200   200   000    Old_age   Always       -       515

The runtime bad block is sort of normal with Seagates. A few is not much to worry about. But I would watch it, and if it keeps going up may be something to worry about.

 

The CRC errors are caused by a bad sata cable or loose connection. Once you fix it, the value never goes back to zero. So the question is, is this something you already fixed, or is this continuing to increase? If it is increasing, you need to replace the cable or at least make sure it is securely connected at both ends.