OK, I managed to clone the OS to the external SD and boot from there using the guide mentioned in the previous post.

Since I am not sure of the integrity of the existing rootfs, I am not sure if some of the problems I still have are attributable to that. What would be nice is if I could get a pristine rootfs and /home from somewhere and use that instead of cloning the one I have got.

Can I get the rootfs onto an SD card using the flashable image from the nokia repositories?

Some of my problems have been fixed:

1. now have /home/opt (after creating the directory) - could not do this when booting from internal flash
2. Can now browse most of the time while on wifi

And some have not:
1. memory errors (although not as often as previously)
2. 3G not working (new problem I think)

Other observations:

1. Installing applications from the App Mgr causes reboots

I think the N900 has a systemic hardware problem. Too bad - I am finding it difficult to cope without it