External 1.5Tb hard disk attached to Synology Diskstation was corrupted for some reason due to the whole DSM 6 nonsense and lagginess and hard reboots. I don’t know whether it was corrupted before, or after, but ends up the Diskstation keeps hanging.

Plug into Macbook, cannot read. Plug into Windows 10, cannot read. Ubuntu VM on Macbook, also cannot.

Finally burned a Ubuntu image onto a bootable USB thumbdrive, boot into Ubuntu, still cannot read. but running fsck /dev/sdd1 managed to fix the corruption and now it is back to readable state.

Oh and it is an ext4 format, which made it more troublesome.

Anyhow I got my files back without having to format and redownload 500gb.