Hello! This is a short guide about how to downgrade from DSM 6.2 to DSM 6.1 after a failed upgrade. I made this mistake myself, so i'm sharing how to fix it!
Your going to need a spare HDD or SSD. Make sure there is nothing on it.
You've installed DSM before, so you should have your synoboot.img from JunsLoader.
Your current bootloader on your USB in now NONFUNCTIONING, you will need to reflash your USB with JunsLoader.
Once you have your new, reflashed USB stick, unplug ALL drives from the motherboard. Make sure there are NO SATA interfaces connected.
Plug in your new, empty drive to the motherboard and boot from the USB stick. (You will need a keyboard for this step) The first option in the GRUB interface is what you are looking for.
Use http://find.synology.com or Synology Web Assistant to find your DSM.
Follow the steps and install DSM 6.1 from a PAT file. DO NOT INSTALL 6.2
IF YOU LEFT A DRIVE IN IT WILL SAY RECOVER. GOTO 2.
Let Synology do its thing and eventually you will have a working 6.1.
While the PC is ON, connect the drives to the motherboard (with you data on it). Under Storage Manager they will pop up and say Unused.
You might need to reboot, at this option. It took me some fiddling around, but eventually Synology will prompt you with a Recover Crashed Partition prompt. Click Yes.
All of your data will appear, and Synology will automatically perform RAID scrubbing.