Which kernel did you yum update Fedora 20 to? There have been a
updated kernels recently which left the BeagleBoneBlack unable to boot.
However, the current one right now, 3.14.2-200, is working OK on a revision
A board. From what I understand, the only difference in revision B is a
bigger built in flash memory (4G vs 2G).
No, Rev C has 4Gb of eMMC, Rev B has a slightly different AM33xx SoC
that's a bit cheaper but wasn't ready when they first released.