I'm running Primo on a 64MB unit so memory constraints can be an issue at times.

Where does the software save the routes to? The device or the SD card? I'm concerned about clogging it up. If the routes are saved to the device is there a change I can make so they go to the card instead?

I think they go in ..\Primo\Save\Tracks, but i always turn them off.

Yes, turn off/disable "tracks"--it keeps a record of everywhere you go, and can turn out to be "a lot" of files. But any Routes you specifically make/plan/save should be in ..\Save\Routes. These you can back up.
In fact, you should probably back up the entire Save folder for those times when Primo may choke due to lack of memory and you have to start setup all over again. Just restore your backed up Save folder, overwriting everything, restart Primo, and you should be good to go.

Excellent tip, Yurbuh !!!
In fact, this is a good tip for iGO too!

Showing my ignorance here but I had no idea it saved tracks by default.
Turning them off seems very wise unless the agenda is to upload a new road to a map creator.
Where do I find the on/off switch? Somewhere in settings? I don't recall ever stumbling across it.

That's a top tip about tracks so I've learnt something useful I didn't realise I needed to know.
My original query was motivated by a desire to pre-plan some circuitous tourist routes with multiple way points for a holiday so I didn't have to mess about with it when abroad. These will reside in save/routes on my card right? Not go onto the device's internal memory?

Everything gets saved inside the Primo folder UNLESS you've setup the folders section inside the sys.txt

Edit: I turn off both options in Trip Monitor, and both options in Log Collection in the settings and i don't get saved tracks.