Changed Startup Disk, need info on Finder

First issue: I am stuck with this system, cannot afford any upgrades.

Second issue is just a request for information.

The now-five-years-old internal HD started to show signs of instability (struggled with backups via SuperDuper etc). So I assigned Startup Disk to one of three external HDs (two 500G, one 160G (I am unable to edit my gear profile here for some reason)).

First time I'd done this, and everything appears to work fine.

I noticed that I needed to replace the Aliases in the Dock, so that apps would launch from the Startup Disk and all work going forward gets done here. I'm content to never access the internal HD again (it's old and too small at its max 160G size).

I further noticed that the Finder and Dashboard aliases are not replaceable as I did the above alias swaps. Is the fact that I've changed the Startup Disk enough to ensure that the Finder is operating on the new Startup and not the old, dying internal HD? Or am I misunderstanding the procedure to begin with?

Just want to completely bypass the internal without disconnecting it outright. Just trying to confirm if I am already doing that.

Am I missing anything basic?

Thanx in advance.