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Yup, it's all down to the behind the scenes detail in the mapping.
If I create a new route today in NT10.3 on my PC it will load straight into the 660 and open up, no problem.
If I open an old route on my PC, say one created in NT10.2 or earlier, it will open in 10.3 no problem.
But, when it is uploaded to the 660 it loads points that come from 10.2, which of course do not match the 660's inbuilt 10.3's roads exactly, so it has to join the via points with straight lines or recalculate.
Apparently there was quite a big deviation in the detail etc. between 10.2 and 10.3, which of course is all invisible to the naked eye.
This would make sense as, if I SAVE AS the old route on my PC (ie. it is now saved in 10.3) and then upload the fresh version to my 660, it will export and open on my 660 with no problem.
OK it's not perfect. As far as I can see the recalculation using 'fastest time' makes no significant change to the route, the device doing its little best to follow the route you wanted it to. As I use lots of viapoints (as opposed to waypoints) when plotting on my PC , may help, too.
Richard
PS But, if I think about it a bit more, what it might not explain is why the route ran fine on my 660 but not on Greg's..... Unless of course between sending Greg the route and the day we went I made some fresh changes that I may have forgotten about