So far so good. Startup drive to external took about two hours for 60GB. Booted off external, all looks OK. Now I'm copying old data drive (internal) to new big drive (internal) using SuperDuper - it's done 88GB in 40 minutes, about 1/3 done.
I'll probably make another backup of the original data drive to the external, before I clone the startup disk back onto it. Just in case the new one croaks. Most stuff is already backed up elsewhere, but you can't have too many backups.
