Unlike the Orion/Kirkwood/Armada based ARM devices, Debian does not provide a pre-packaged kernel which supports the MV78100 SoCs. Although the device can easily run Debian's "armel" packages it requires a customized kernel to boot. Modern kernels also have issues with where in memory the device loads initrd images by default.
Installer files can be found in the armel section of: https://github.com/1000001101000/Debian_on_Buffalo
Note: these devices have strict requirements for how the /boot partition is formatted. Currently, they must be formatted before starting the installation (either manually or by re-using the stock firmware's /boot). The "No, keep existing data" must be selected during the install or the device will fail to boot. The exact requirements for a /boot partition can be found under:
Otherwise, the install process is the same as the one described in: