Update
I was finally able to restore my device by restoring from an entire disk image (not just the rootfs raid1 partitions). Not sure why restoring the rootfs by itself never worked, I’m guessing I must have messed something else up. The image I ended up choosing was the one provided by Yo_Bubba in this thread (mycloud2tb_transmission_v03.04.01-230.7z). I was wanting to install transmission anyway, so that made it easier.
I ended up just using dd to restore the image onto /dev/sdb (after backing up my files):
dd if=mycloud2tb.img of=/dev/sdb
Then I just used gparted to resize the 2TB partition to 3TB (filling the rest of my disk space).
