Beaglebone Buffer I/O issues

Question: Where does debugfs actually reside? On the microSD or on the beaglebone hardware itself?

mount should tell you that:

debugfs on /sys/kernel/debug type debugfs (rw,none)

The kernel provides it, it's not a disk backed file system.

tmpfs (if that's what you meant to ask about) resides where ever you
want it to. Often /tmp is a tmpfs, for example.