Be VERY careful when running apt-get upgrade with cssu-devel enabled. It is highly likely it will install the package kernel-modules but not actually install the newer kernel. If that happens your N900 will not be able to properly boot (even backupmenu will not be able to restore any backups).

This was what caused my own N900 to fail before causing no end of problems (which were finally solved when I was able to get an external battery charger to charge my battery and then reflash the root FS before restoring my BM backup and moving on from there with installing the very specific packages I actually wanted to install on top of CSSU-testing)