'The disk you inserted was not readable by this computer' when connecting external hard drives

I have a Synology NAS that has failed and I won't be replacing it. It had two 4TB Western Digital Red hard drives. I've bought a dual hard drive disk reader on Amazon and inserted both hard drives but my Mac shows this error (below)

I want to be able to retrieve our data as it has family pictures and a lot of other valuable data stored on it.

When I check both disks with the DriveDx application they pass all tests, however I still can't seem to mount them.

Solution 1:

Depending on the age of the DiskStation the HDs should either use EXT3/4 filesystem or Btrfs filesystem. Both filesystems are natively not supported in macOS (hence the "not readable" dialog). If the NAS HDs are formatted in EXT3/4 you can try the EXT filesystem package for macOS by Paragon software (trial is available). https://www.paragon-software.com/home/extfs-mac/

If the filesystem on the NAS HDs is Btrfs, no luck with macOS. There is no Btrfs filesystem "driver" for macOS available. You can then only try to mount the drives on a system capable of reading Btrfs (another Syno NAS/Linux box etc.).

Solution 2:

I want to be able to retrieve our data as it has family pictures etc. and a lot of other valuable data stored on it.

You won't be able to mount the individual drives because Synology employed (as a default) a technology called SHR (Synology Hybrid RAID). So, to recover anything, you will need another Synology unit. Short of having a second Synology unit, your data should have been backed up.

SHR is an automated RAID management system that makes storage volume deployment easier than traditional RAID systems. SHR will allow users to handle RAID management, expand storage, and maximize storage capability even if they do not have a fine grasp of various RAID levels. SHR allows for 1-disk or 2-disk worth of redundancy - meaning the SHR volume can suffer up to two disks lost, and the data volume will still be available for use. Note that a RAID volume (either Classic RAID or SHR) is not a backup system.

Even if using "classic" RAID, you still woudln't be able to mount the individual drives; they would be need to be mounted as the entire array (and this is before you get to the filesystem).

It can't be said enough that RAID is not a backup. It's a fault tolerant device, not a disaster recovery device.

My suggestion is to obtain a similar Synology unit (secondhand via eBay is fine), rebuild the array then archive them to a USB drive.