You should be able to have the OS on an external drive and the apps on the internal drive. I've done it before and I don't remember having any issues.
It would also be possible to have everything on the external drives (apps included) but that means taking time to clone the drive including all apps. I have one external drive with the last 3 or 4 OS versions for all kinds of situations like this.
Thanks, that's encouraging and sounds like the easiest option to start with. I have considered cloning but I'm wondering if the method I've read about is considered the best:
1 – clone the Mac drive to an external drive,
2 – then update the Mac's OS to, say, El Capitan.
3 – test legacy apps and if they don't work wipe the Mac drive and restore the cloned backup to it.
Have I understood that correctly? It seems to me that a less risky approach which not would involve wiping the Mac drive would be to:
1 – clone the Mac drive to an external drive,
2 – then update the clone to El Capitan.
3 – Boot into that, and if the apps don't work abandon it and go back to the untouched Mac drive.
Is that not a better way of doing it, or am I missing something?