N900 won't boot: gives bright LED/"Nokia" screen for 30 seconds, then quits (likely not a low-battery problem)
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When I press the power-on button, the LED lights up bright-white,
a brighter-than-usual white "NOKIA" screen displays (with a brief vibration), and both continue for about 30 seconds, then the phone goes dead (this can be repeated).
If I attach the N900 to a car-charger, *the LED glows bright orange and a dim (unlit) "NOKIA" screen displays, both for about 30 seconds (probably with a vibe), then appear to go dead for about 2 seconds, then the dim screen repeats this pattern (the LED behaves variously) indefinitely.
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Some possible clues:
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Three times just recently, turning the phone on brought up a bootmenu page, offering 'nand boot' and another boot-choice (this is not something that otherwise appears frequently);

Last week I apt-get-installed wget and debootstrap;
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About a year ago I had much the same booting problem for a while, except that booting would usually succeed after three or four attempts; the problem eventually went away. I suspected it may have been caused by overcharged batteries (sometimes I charged them externally, but not lately);
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I charge with a car-charger connected to a 12-volt battery, sometimes in parallel with a 17-volt solar-panel (haven't noticed any problems, except possibly the N900 then charges more slowly);

I download lots of files (mostly htmls with associated folders, pdfs, videos) for offline viewing (malware? ...);

Over the last year or so, as internal and SD memories became fairly full, the N900 has become increasingly sluggish, particularily the Folder app: on pressing a menu button, it can take 15 seconds before there's any visible response.

I'm searching this site and about to try the Flasher, but when I run it, a commandline box shows for a second and disappears. I've seen a simple fix for this but forgot what it is? ...

Sounds like your charger does not give enough power for the phone and battery is empty/damaged. Do you have the original 1,2A Nokia charger?

You are really sure it is not the battery? If you are I would probably have tried to flash my device as well with these (vague) problems.

Your post does seem to miss some information to give you any advice on flashing though. On what OS are you? What flasher did you use - and which tutorial / post did you use as guidance?

It sounds like tfj is trying to start the flasher by double-clicking in some kind of GUI. That won't work,tfj. You need to start it from the command line, typing some parameters. Have you followed this guide?

http://wiki.maemo.org/Updating_the_tablet_firmware

The batteries are pretty new. This is the Maemo 5 OS; I'm on a public Windows 8 Pro machine. The Flasher I was going to use is the 'flasher-3.5 for windows', I was following one of this site's threads. I'll need to read-up on flashing and will try the wall-charger and command-line next week.
When I hold down the u key and connect to the PC, the bright orange LED and unlit 'NOKIA' logo and a USB-symbol come on continuously, I guess it's charging; the PC recognizes it as a Nokia "Wireless Communication Device" in 'update-mode' and says it has the right drivers.

pichlo gave you the correct link.
Read, understand, follow.

The flasher is not a one-click GUI but installs as a DOS console tool. You need to start the ('flasher') console and type in the commands manually.

Mainly your prob will be 32/64bit, please check (also given in wiki link):
http://talk.maemo.org/showpost.php?p...0&postcount=20

I noted that the (2) batteries are newish, but maybe their internal charging-circuitry has locked-up?
I'm not sure, but when the N900 is turned off, the battery may drain 'rapidly'.
When connected to a car-charger, there's 60 mA leaving the battery when the yellow LED glows, and 220 mA when it doesn't , so the phone apparently is charging.
The wall-charger gives a similar result, except the initial 30-second yellow-LED doesn't display.
Before trying Flasher, are there any other tests I could try? - Would RecueOS be useful?
I understand flashing the FIASCO image won't erase the /home/user and /home/user/Mydocs files, but what will it erase? /home/opt? I don't recall what is on the 240? Mb partition.

never flash N900 with empty battery .. even if said battery is new!

you need to charge that battery somewhere else (universal charger, Nokia 5800, etc.)

also, N900 will not draw a lot of charge current on charger that does not follow the specification to the letter, i.e. a charger needs to have a short on the D- and D+ pins. If it doesn't have that, no matter the rating, N900 will draw less than 500mA on it

You need to flash kernel separately. Asume you're in the folder where you downloaded the flasher and Fiasco image, put the name of your RX-51whatever.bin instead of <>.
flasher-3.5 -F <firmware-image> --flash-only=kernel -f -R
I personally had that problem after some unclear update.