You can see if there is a problem with back cover with this command in terminal:
# dmesg | grep mmc0

For example, now when I have Mugen 2400 mAh battery and its design flawed back door attached, it shows:
"[ 47.081634] mmc0: cover is open, card is now inaccessible"

I have a reclamation ticket waiting on Mugen. They promised to contact today, but nothing yet.

Hah, I just tried with a magnet and back door removed:
# dmesg | tail
[69147.354370] slide (GPIO 71) is now closed
[70042.612609] kb_lock (GPIO 113) is now closed
[70042.885528] kb_lock (GPIO 113) is now open
[71874.541870] mmc0: cover is closed, card is now accessible
[71875.041870] mmc0: host does not support reading read-only switch. assuming write-enable.
[71875.041931] mmc0: new high speed SD card at address b368
[71875.050201] mmcblk1: mmc0:b368 UD 968 MiB
[71875.050567] mmcblk1: p1

.... Then I took the magnet away:

[71893.135955] mmc0: cover is open, card is now inaccessible
[71893.331512] mmc0: card b368 removed

I'll say, it is a design flaw also from Nokia. Because of those magnets, there is no useful digital compass in N900 like in N97.