How to move data from HDD with bad blocks to new one, mounted in place of faulty?

[sarilian]

Hello everyone,
I have 2-bay Asustor NAS (5002T) with 2x Single drive setup (yes, I'm a cheap ass!) using 4TB WD Red HDDs.
I noticed that one of them (drive 1) started having bad blocks, so I have purchased new drive to rescue data from the faulty drive.
I have mounted new drive in place of faulty one and set it up. Now, I wanted to connect the faulty drive to rescue data from it. When I connect the drive to USB or eSATA ports, it recognizes the drive, shows partitions (4) and all the other details, but tells that data is inaccessible as the format is wrong (available data 0B of 0B). When I replace new drive with an old, everything looks good and I still can access my data - files looks unaffected.
How can I connect old and new drive together so I could copy the data from old drive with bad block into new drive? When I tried to replace drive 2 (which is working fine) with old drive 1 (with bad blocks) to copy over from one drive to another - everything goes bad and cannot log into the ADM via web browser.
Cheers,

[orion]

If you know how to use "mdadm" command through ssh, you may be able to recover data in case that you connect failed disk to NAS from USB port or eSATA port. (It's still software md drive if it's a single disk volume.)