I just had the exact same issue, yellow light after installing Transmission. I debricked my MBL Duo using the script mentioned in mybookworld.wikidot.com / forum / t-607715 (make sure you remove the spaces in that link, I can't link directly because my account is new) it keeps all your data on the disks, and you can access the data afterwards using SSH. If you run the script, make sure you only have the A disk connected to the computer.
After recovering my data I did a full factory restore, just to be sure.
Another method to get your data off is a bit more cumbersome, but also worked for me (I did this before running the script mentioned above). First attach both the A and B disk to empty SATA slots. Boot your computer using a Ubuntu live disk, once that is done - mount the disks using mdadm and get your data off using debugfs. You can use the rdump command in debugfs to copy folders to another disk. (trying to find some related links for this, but can't seem to find any right now…)