Thanks, I could verify on a friend's PC (Vista) meanwhile that my N900 does operate correctly, it does expose both the SD card and the home directory and Vista shows it as two separate drives.

The "Probe multiple LUNs" kernel config is something I'll have a look at, thank you very much, patstew!
I'm not at home right now but I'm sure I wouldn't have discovered that as a possible cause for my problem. I'll let you know if it was.