It's exactly what the error message says: "device /dev/sda1 does not exist" yet.
It may take a couple of seconds for the device nodes for the partitions to be created, so you are trying to mount it too soon.

This kind of problems have been discussed here a lot. Search this forum for "usb mount delay" or something of that kind.

No. The data is usually spread throughout the flash and you would have to cherry-pick individual blocks.

You could just copy the file system contents using tar or cpio to another Linux machine and create a new UBI image there.

Yes.

No, JTAG is not necessary to access the serial console.

The other way around: 4 pins UART, 8 pins JTAG.

I'm not aware of any product that you could use out of the box (this is not to say they don't exist).
What I meant is that you could make one yourself. Buy the components, solder them, etc.
(see http://plugcomputer.org/plugforum/index.php?topic=1678.msg10464).
Or you could modify a product with a similar function.
(see http://plugcomputer.org/plugforum/index.php?topic=1797.msg10821).

Yes, it is wrong. What is labeled on the picture as "Mini USB-Port" is actually the JTAG connector. The other one labeled "JTAG UART RS232" is UART (not JTAG, not RS-232).

Are you sure you are talking about GuruPlug Server Plus? It does not have a mini-USB port. It has UART connector (for the serial console) and JTAG. The "official" way of using those is to purchase a JTAG board specially made for GuruPlug. Just a cable is not enough. If you only need serial console, but no JTAG, you could make a cheaper converter from UART to RS-232 or USB. If you search this forum I believe you will find a number of posts about that.

I believe this question has been asked a couple of times before and, as far as I know, has not been answered.
It appears that uaputl is not open source.
In theory you could ask Marvell, but according to what is written on their web site, they won't even talk to you until you sign an NDA.

Your remarks about the quality of the PCBs apply to GuruPlug as well. The main PCBs (two of them) look nicely designed. But the power supply PCB reminds me of those hand-made amateurish boards I used to make for a hobby in the old days.

The last release of the official (not Marvell) U-Boot (2010.06) contains those fixes. I have a binary for GuruPlug.

I don't see any reason whatsoever why the root file system can not be read-only (and I believe it should be). Besides preserving the flash, it also protects the data from accidental damage by buggy software or, more likely, by a user's mistake, such as "sudo rm -rf /" . I've been using this for many years on my home router.

The recipe is quite simple: mount tmpfs on /tmp and unionfs (or aufs, if you prefer) on /var. The unionfs/aufs mount should contain two branches: read-only - the original /var, and writable - another tmpfs mount somewhere (I use /mnt/var for that purpose). Using unionfs/aufs relieves you from the burden of creating all necessary files and directories in /var upon each boot.

Not that I'm aware of. Does it really have to be a plug computer?
If you are looking for a device with low power consumption there are lots of those based on x86-compatible CPU (such as AMD Geode LX or Intel Atom). Although they are considerably more expensive.

Sheevaplug uses Ubuntu for ARM. Skype only provides binaries for x86/x64 (and no source). This means you cannot run Skype on Sheevaplug.

The 88F6281 SoC used in SheevaPlug (and GuruPlug) belongs to Marvell's "kirkwood" family (sometimes abbreviated as "kw"). ARM architecture version is "armv5te" and the CPU type is "arm926ejs" (sometimes spelled "arm926ej-s").

If you are talking about U-Boot code then the directory you are looking for is "arch/arm/cpu/arm926ejs/kirkwood/". Although I can't imagine why you would want to change something there.

You've got the idea. There isn't much of configuration though. Upon reset BootROM checks the state of a few pins of the SoC to determine where it should load the second level boot loader from (NAND, SPI, whatever else). This choice is often hardwired on the PCB, since most real-life devices only have a single place to boot from. In that case there is no configuration at all.

You may be confusing yourself by using a vague term "boot image", which can mean about anything.

BootROM is a small piece of code located inside the SoC chip. It is put there during manufacturing. It is not a part of U-Boot, in fact it is unrelated to U-Boot at all.
Yes, the function of BootROM is to load and start what you call "second stage bootloader", which is usually U-Boot (but does not have to be).

One of the first things I did when I got my plug was decreasing the MTD partition size for kernel. I think 4MB is way too much. If you are a bit conservative about what should be built-in (as opposed to a loadable module) then you will usually end up with a kernel under 2MB.

Although, I did not even try to play with the "mtdparts" boot option, I just changed the default partitioning in the kernel source, as I do not intend to change it again any time soon.