My two cents for further encouragement: I just successfully installed Arch on my TonidoPlug2 following the instructions in this thread and at http://archlinuxarm.org/platforms/armv5/tonidoplug#qt-platform_tabs-ui-tabs2, using the cable and the pinouts described at http://www.tonido.com/forum/viewtopic.php?p=19064#p19064.
Just to consolidate the information into one place, here's the full process I did:
1. With an empty SATA HD in the drive bay and a USB serial cable connected, boot Tonido. (Strictly speaking, the HD doesn’t need to be empty, it just must not have a /boot/uImage file).
2. Verify that the serial connection works while booting, i.e. you see what is printed and are able to log in after the boot is done. I used PuTTY, with the USB serial connection as COM3 (115200 baud, 8 data bits, 1 stop bit, parity None, flow control None).
3. Prepare the HD and install the Arch Linux files on it as instructed here: http://archlinuxarm.org/platforms/armv5/tonidoplug#qt-platform_tabs-ui-tabs2
4. Reboot with the serial console active the whole time. When you see the “NAND:” line, hit Ctrl-C. You should drop into a Marvell>> prompt. Type printenv, copy and paste its output for safe keeping somewhere. (help gives you a list of commands, help command for more info on a command).
5. Type the following commands into the command prompt:
$this->bbcode_second_pass_quote('', 'M')arvell>> setenv mainlineLinux yes
Marvell>> setenv arcNumber 4101
Marvell>> saveenv