Please, how to formate microSD in N900? I cant see formate possibility in file manager or memory tab either.

Just tap and hold on the card in File Manager and you should get a popup offering several options including formatting.

I wasn't able to format my microSD in the device (was trying to format as ext3 through xterminal) even after unmounting I was always informed that the device was in use.

I ended up having to format the microSD on my laptop.

You can also reformat it in USB mass storage mode. The N900 exposes the micro-card (and the MyDocs partition of the eMCC device) as raw usb drives to the PC. You can repartition/reformat either one.

BTW the back of the phone has to be closed before the MicroSD card shows up in file manager.

If you did a cd to a microSD directory it is in use (witt any program). If you leave the directories on the microSD it is not in use (with all programs).

I had the following thing which also did not allow me to unmount my microSD:
I opened a terminal. I went to a microSD directory. I changed to root which is in /root at the beginning and then I tried to unmount - didn't work. Then I exited the root shell changed the directory to /home for the normal user and then I was able to unmount - maybe you had the same problem.

Thanks @timwatt !!!!!
I was trying like hell but wasn't putting the back cover and trying trying trying with it, read your post and this now works like a charm.

Thanks for posting this
Anmol Saraf