I get this annoying error message 8 out of 10 times when I try to connect my N800 to my Windows XP desktop PC, and I have absolutely no idea how to get whatever the heck the OS is doing on the card to stop, unmount the card and let me get on with what I need to do urgently.

This has frustrated me to the point that I was 2 feet from throwing the N800 out of my 2nd floor window. Before I lose it completely, does anyone know how to get the OS to leave my internal card alone?

This is happening even with a fresh boot and no applications running

The OS does not like you touching the internal card which is the one you are running it from or swap file in on it. You would have no problem with an external one. Try to remove swap file see what would happen.

To add: the swap file has been deactivated too...

And I'm still getting the same problem. Odd thing is, when I insert the SDHC into my Vista notebook's SD slot, it is not recognized. However, I have had no problems with my other SD cards.

What I would like to do now is to format the SDHC in Vista and start afresh

Well, I don't know why it's busy. But my approach is to have a big (16GB, for now) internal card, and a small (128MB, came with) external card. I use the external card for files I might want from the computer.

Been doing that since OS2007, when I had no problems with the internal card, except when I was playing music from it. So I changed for convenience's sake.

Regarding 'SDHC does not work, all other SDs do', I suspect it's a case of an SD (not SDHC) adapter. So you won't be able to format it in Vista. I don't see any reason to suppose there's a filesystem problem here, just OS2008 doing something dumb.

I have seen this happening to me few times. In my case I do not connect N800 to PC but when I was trying to remove the internal card.

One workaround is simply shutdown and restart N800. The card should become "free" after restart, after the metalayer crawl has finished its scan thing... (If you have a lot of media files on the card, this may take a while.)

Second option, I just discovered recently, happened when I tried to remove the internal memory card: N800 said the card is in use and offered option (button) to shutdown programs using the card. I closed the battery cover again and clicked that button. After that I opened the battery cover again and it no longer complained about "memory card in use"...

It must be a corrupted file on the FS. I would boot from flash and do a fsck -a on the SD partition.