If you follow through on the UBIT install instructions to install Arch Linux ARM install Linux to NAND, you will have a working backup system, even if a subsequent debian install doesn't go according to plan.
UBIT provides sensible u-boot environment parameters to allow booting from external devices without having to alter anything at U-Boot level.
You just need to provide:
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... on your ext2 boot media. This can either be the first partition of the first USB drive or the first partition of the first SATA disk. Both will work in the UBIT U-Boot boot sequence.
If these are available they will be booted instead of the UBIT NAND-installed kernel. However...
U-Boot will set the arcNumber parameter to 3089 (the registered value for the GoFlex Net) and any kernel must have the GoFlex Net code in order to boot from this parameter and have working SATA ports. At the moment the only maintained kernel providing this is the UBIT kernel. Other kernels will not boot and/or provide SATA port functionality.
If you have installed UBIT Arch Linux ARM install Linux, you therefore don't need to put /boot/uImage on your external media. It will always be found on NAND memory, but the boot behaviour will still favour booting to external media just as long as U-Boot finds /uInitrd on an external drive.
For external boot, the UBIT U-Boot sends the kernel the parameters "root=/dev/sda3 rootfstype=ext3". If you have both SATA and USB devices plugged in, the first SATA drive will be /dev/sda. If you just have a USB drive, it will be /dev/sda.
Mixing and matching USB and SATA drives is a source of much complication, however at this stage if you unplug all devices, the system will boot into Arch Linux ARM install Linux, giving you a rescue system at the very least.
Debian's boot ramdisk does quite a bit to set up the file system and I haven't worked with it to see if it is compatible with the parameters I am sending from U-Boot. This is the starting point for GoFlex Net Debian pioneers.
1. Install the UBIT system in full
2. Set up a rootfs on /dev/sda3 (either SATA or USB, with SATA taking priority)
3. Set up an ARM imaged Debian ramdisk (uInitrd) on the first partition of the boot media (USB or SATA doesn't matter)
4. ... after that it is up to you...