I’m sorry for the failure you have in your disk.
EasyNAS is a sophisticated GUI for a linux box. there is an option to mount via command line.
in the console select shell
and type: sudo su –
you will gain root access to the system.
try to look is btrfs shows the drive, type:
> btrfs fi show
mount the filesystem manually:
> mount -t btrfs /mnt/fs
please let me know what you got.