Thanks ndi, your last message sounds very interesting. I'm not much expert in english language, and I've understood partially each one of your answers:

• I don't know what is "PR"
• I don't know about which "radio modem" are you talking, or why in this context, same as some GSM or why is related to the question (doesn't it work with a normal software driver as any other device?).
• "reflashing both the root and the EMMC"... Doesn't the EMMC the internal place where the root is put?
• The lock code? I've read the Nokia N900 hasn't a lock-in. Is it stored in some else place?
• "modified PR 1.2 there's no telltale"... I don't understand the expression, sorry.
• "should you fry your device with OS"... are you talking about when replacing a kernel? The Maemo Linux kernel or another kernel? Which are the flashing movements with more possibilities to definitely brick the device? What is "OC"?
• The Nokia N900 has water coolers?

Please, could you explain your answer to make it easier?
Thanks.

Stands for Public Release. OS is version 5, so first version is 5.1.0 AKA PR 1.0. Then 1.1 is called PR1.1. Each one is a version of the firmware, released by Nokia.

Yes and no. The GSM radio is its own chip, independent from the OS, running a firmware, much like a modem in a PC runs a firmware. The installation of 5.1.2 (PR 1.2) upgraded the chip's firmware. That firmware can't be downgraded, and the driver in 1.2 no longer works with the firmware in 1.1 and the other way around.

Once flashed to 1.2, if you revert to 1.1, 1.1 can't use GSM because the chip is from the future.

Short version, only flash in same version or higher, never backwards. Or GSM doesn't work.

No. N900 has a 256M internal fast flash, where the OS resides. This is root. The EMMC has a 2G partition for "applications", called "home" and the rest of (30?) as "documents", that stuff that gets mounted when you plug in USB. Mainly, because it's FAT and it's easy to share a FAT over USB. The root and home are only visible to the device and via SSH or other services that do this on purpose (ftp, etc)

Home is also where apps install, configs are stored, etc. Not rewriting that could cause things like passwords, settings, etc to stick.

Not that network lock, the lock code when you lock your device so it can't be accessed by others, you know, 12345. Power button> lock device.

A telltale sign is a sign that tells the whole tale (story) when observed. E.g., corner scratches are telltale signs of dropping the device.

I assumed you want to overclock your device. If you don;t know what OC is at first glance, don't do it. And yes, I am talking about replacing the kernel. That is one of the few things you would try to ... er ... change when returning your N900.

Why else revert the thing? Contacts, images, messages can all be deleted from the UI.

If you want to. No, I was referring to PC components that have been obviously overclocked. But, since you don't seem to want that, nevermind.

Sure. Here you go.

Thanks ndi, you've been very comprehensive.

I like the possibility of overclocking my device, such as other things, but I prefer to learn everything about the device before doing this or any other.

Better than "Oh! Curiously this works!" I like better to know why some hack works, and what (and why) are the risks before customizing the system.

Nice to know the partition architecture: fast 256MiB flash memory for (read only?) root, and 32GiB for random write/access filesystems {2GiB for software and config + 30GiB in FAT for user documents}.

But now I discover that some versions of released firmware include independent firmwares for devices in the device (as GSM modem). Does it mean that GSM modem's firmware cannot be backup in the same way?

a) root is read-write, and software can be installed there, as in normal Linux. However, because space is limited, even though root is compressed, they install in the 2G partition, mounted in /opt. The process of moving off root and in the 2G partition is called "optification/optifying"

b) You can't back up the radio driver, correct. Why older versions don't downgrade the driver I don't know.

c) There's also the card, bringing the grand total to 50G ish, it's mounted in /media/mmc. 16 G max because that was the largest this far, but people report no problems with 32G microSD. Why would anyone use more I don't now. But, why not?

d) Also, there's a partition you can't see, for swap (754M).

e) Most of this info is in the wiki. It's a good read at your own pace, includes OC, custom kernels without OC, well, everything you want to know about N900. (Wiki.maemo.org)

Ok, "OC" = OverClocking.

Do other GNU installable systems (Debian, Ubuntu) have problems to use devices as GSM modem? People who install other OS, does it replacing Maemo or adding another root partition + boot selector?

GSM modem driver and interfaces are closed (and proprietary I think). Good luck with that.

Another utility for bi-directional flasher: backup user partitions before reflashing when bricked.