All of the drivers and subsystems output status messages to tell you what's going on. The command 'dmesg' dumps the kernel's log buffer. What you see on a line is a time stamp' [ dddddd.dddd]', the driver or subsystem name, colon, and then the log message.

In this instance what we are interested in is the mmc drivers (MMC is the original form factor of SD cards). There is only 1 reader in the N900, so it's number will be 0 (zero). So the string of interest is 'mmc0'. And this being a POSIX computer, you have most every tool you need. So filtering is super easy.

dmesg | grep -i 'mmc0:'

That command will show you all of the log messages that the card reader driver outputs. You don't need to be root to run the command. And it will not hurt anything at all.

And here is a sample.

This shows that I removed my back cover, and putting the cover back on, and the system recognizing the 8G SD card. The last line shows the kernel reading the parition table and seeing the 1 partition on the card. The actual mounting of the card into the file system is not done in the kernel. It's done in what's called user space (outside of the kernel). So those event's won't show in the kernel log.

If you can see similar entries for you card and still not visible on the N900, the file system maybe corrupted such that the N900 can't/won't mount it. Then I could copy all of the stuff from the SD card, reformat it, and put the contents back on there.