Two disks apparently failed in ReadyNAS Pro 6

In future in the unfortunate event you run into such a problem again (it sounds like you could be an IT service provider that has several clients so there's a fair chance you might at some point) it would be better to use a tool designed for cloning failed disks. Of course it would be even better to have an up to date backup. Disks can and do fail and it's unfortunate.

Make sure email alerts are configured correctly so that if disks give SMART errors someone will get emails.

You may also wish to get this client to use X-RAID2 dual-redundancy or RAID-6 going forward. This gives some protection against two disk failures (but there are other reasons to have backups than disk failures e.g. accidental file deletion, fire, flood, theft, other hardware failure etc.). When you factory default (wipes all data, settings, everything) the box with four or more disks installed there will be a few quick checks and then a 10 minute countdown will commence. During this countdown open RAIDar, click setup and choose X-RAID2 and select the dual-redundancy option and confirm your choice.

The following instructions have been posted a fair bit on the community. You don't need to use a Live CD for this, but if you don't regularly run Linux then a Live CD is a good option:

"Use a Knoppix 6.2 Live CD for this guide. Can be found at http://www.knoppix.net 

Using dd_rescue command allows you to copy data from one drive to another block for block. This is especially useful for recovering a failed drive. Often when a drive fails, the drive is still accessible, it has just surpassed the S.M.A.R.T. error threshold. dd_rescue allows you to ignore the bad sectors and continue cloning the bad drive to a new healthy drive. 

1) Connect your old drive ["failed" disk] and new drive to your PC. 
2) Boot up using your Linux live CD. 
3) Launch a terminal window. 
4) Run fdisk -l to make sure the system sees both of the hard drives. 
5) Run hdparm -i /dev/sdx on both of the drives to find which drive is your source drive and which drive is your destination drive. 
6) Once you know which drive is which you can start the clone process. 

dd_rescue /dev/sdx(source disk) /dev/sdx(destination drive) 

7) You will see the process start, just keep an eye on it, it might take a few hours for the clone job to finish, depending on the size of the drive. 

Once the process is complete, there will be no notification, the transfer will just stop and you will see the terminal prompt again. 

If you see a lot of errors or see that there is no more data being shown as succxfer: it means the drive got marked faulty by the kernel. At this point reboot the system and make sure you know which drive is which again, as it is possible the lettering might switch. Run the dd-rescue command again but this time with -r option. This will start the cloning again but this time will start from the back of the drive and will make sure to get the data that has not been cloned yet."