In a few hours, my new N900 is going to arrive via UPS.
This topic is a log of what I will do/have done to it, for two purposes: One, if I screw things up, I will be able to work back through this and restore things to how I had them before. Two, this will allow you, the curious N900 owner to copy any bits that I got right.
Note that I am running a Linux(x64, KUbuntu 9.10 & Mint 8 KDE) system, so my commands will be Linux based.

Items with a * before them have -NOT- been completed yet. YMMV.
Instructions will be in [b]Bold[/i], comments/results in normal text.

Here goes:

1. Unbox N900, make sure everything is there
Found N900, charger, USB cable, spare stylus, and getting started guide(which I haven't opened).
2. Pull back case off N900, remove battery, make sure there is -no- sim. If there is, remove it.
No sim found, battery dead. Charged battery for a few minutes.
3. Boot up N900. Check what firmware version, and if there are any files left by previous owner. If any files left, back them up.
Firmware version is "10.2010.19-1.002". I didn't see any files left behind(Though I saw 1.5GB of 'cities' data). Backed up settings, copied everything to my laptop.
4. Download PR1.2 on PC along with flasher
Powered N900 down, pulled out battery. Entered IMEI number(with one digit changed) into Here.
Downloaded RX-51_2009SE_10.2010.19-1.002_PR_COMBINED_002_ARM.bin.
Downloaded maemo_flasher-3.5_2.5.2.2.tar.gz.
5. Flash PR1.2
Unzipped the flasher via Dolphin.
Entered the flasher DIR, pressed F4 to bring up a terminal window.
Ran "sudo chmod +x ./flasher-3.5"
Ran "./flasher-3.5 -F ../RX-51_2009SE_10.2010.19-1.002_PR_COMBINED_002_ARM.bin -f"(image in parent directory).
Got "Suitable USB device not found, waiting." message in terminal.
Held 'U' key on N900, plugged USB cable in. Got USB icon, released U key.
Got "Error claiming USB interface: Operation not permitted" in terminal.
Checked that modules 'cdc_phonet' and 'phonet' were not loaded(these modules caused this error with my N770).
Ran "sudo ./flasher-3.5 -F ../RX-51_2009SE_10.2010.19-1.002_PR_COMBINED_002_ARM.bin -f" with N900 still plugged in and in flasher mode.
Device flashed properly.
Unplugged USB cable and rebooted.
6. Install software: vim, rootsh, mplayer.
Added Maemo Extras repository.
Added Extras-devel repository.
Note: be careful to put 'fremantle', not 'freemantle' as I did the first time.
Installed rootsh.
Ran xterm, typed 'sudo gainroot'.
Ran 'apt-get install vim mplayer'
7. Rename /usr/bin/cherry to /usr/bin/cherry_old, make shell script:

Code:

#/bin/sh
return 0

Save as /usr/bin/cherry.
Ran 'cd /usr/bin'
Ran 'mv cherry cherry_old'
Ran 'vim cherry'
Pressed 'I' to get into insert mode, typed '#/bin/sh*enter*return 0'. Typed '*escape*' to get out of insert mode, then ':wq' to save and quit. Exited xterm.
7. Mess around with the device, test it out and check stability.
Haven't had a crash yet, device seems responsive. But, I'm mostly skipping this step.
8. Install nbench from Here.
Doiwnloaded the three files via my laptop.
Copied them to N900 via usb to N900/nbench
Opened xterm on N900, cd'd to /home/user/MyDocs.
Ran 'mv ./nbench /home/user/' Got several 'error' type messages about owner not being able to be preserved(vfat issues?), but the files moved.
Ran 'cd /home/user/nbench'
Ran 'chmod +x ./nbench', then './nbench' to start.
9. Figure out a way to backup the n900. Test multiple methods, figure out which works.
I've messed with tar and excluding the dirs I don't want. It seems that the busybox version of tar only takes one 'exclude' variable. I installed 'tar-gnu', and then tried running the same command with 'gtar' instead. It seems to work. Currently I'm running this command as root:
First time I ran it, I got a 'write error' after a couple minutes - probably caused by trying to write to the mmc when it was in USB mass storage mode, though the SD card wasn't mounted on the PC. Switched to charging mode, trying again.
This time it worked fine. No errors whatsoever.
I backed up a second copy of my archive(~250MB) to my PC, and flashed the 1.2 firmware to my N900. Now to see if I can't restore it properly.
I opened Xterm, and ran "tar xzvf /media/mmc1/backup.tar" from the root directory. Errored out - no permissions.
I then opened the App manager, checked for updates(Note: On wifi here), installed 'rootsh'.
Ran 'sudo gainroot', then 'tar xzvf /media/mmc1/backup.tar'.
Hm, it doesn't like the '/syspart' files. I'll remove them from the archive, try again.
Trying one section at a time using "gtar xvzf /media/mmc1/backup.tar --wildcards bin/*".
Everything worked fine until I got to 'usr'. The phone evidently crashed and when I saw it it was in flash mode. It wouldn't reboot fully after that - had to reflash it again.
Tried again. Does -not- like writing to 'usr'.
So, for backup, the tar method works fine. For restore, not so much.
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New method:
Open app manager, install 'gainroot', 'wget', 'coreutils-gnu'
Open xterm. type 'sudo gainroot'
type 7 commands:
Bootmenu works, but only allows internal flash or a "recovery boot" with network only access - no terminal!
Yes, it may be possible to backup over the network, but this isn't a solution.
Hm. If we can boot Debian off of a floppy and get a command prompt(on a pc), we ought to be able to load a tiny debian/otherOS image onto the SD card or eMMC, then boot into that and fix things. All we need is a command prompt.
Props to anyone who can recommend such a solution!
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Of cource. The one solution that should never work... seems to.
I went back to basics and tried a "dd if=/mnt/mtd5 of=/media/mmc1/backup.img bs=4K", let it back up, then copied it to PC, flashed the device back to defaults(to make sure it works), then used flasher to load that image.
Upon doing so:
Not so sure thats good, but not only did it still boot afterwards, it actually had all the programs I had installed, and everything worked!
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Ok, lets try a new, possibly the best method yet, using a custom bootloader app. Thanks to 'hawaii' for this:
I'm gonna try using the process outlined Here, then improve the rescuemenu shell script to have a "backup" option.

First, using my N900, I opened xterm, got root, then ran 'apt-get install bootmenu-n900' (make sure that 'extras-devel' is enabled, or install the package manually).
.......

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*10. Run nbench for 30mins, using a script if needed to keep it running. If the device stays running during this, proceed with OCing. If not... test again, and if it fails, see about returning to Nokia.
*11. Install D-THEME black.
*12. After testing and playing around with it, proceed to OCing using resources from [url=http://wiki.maemo.org/Overclocking]Here[/ur] and the Maemo Talk forums.
The plan here is to OC it, figure out the maximum stable frequency with one of Titan's kernels, then see about manually adjusting voltages down on each frequency until it crashes(during nbench tests), then bring it up one notch. I figure the values I arrive at will be close to Titan's levels, but they may not be exactly the same, what with tolerances and all.

--In progress--