Since new in 2012, the laptop has occasionally powered off without warning, no blue screen or error message. Can go for a month or 2 without happening, or 2 or 3 times on a very bad day. No pattern to it. No solution ever from Samsung tech support. Not repeatable enough to justify return for hardware fault investigation. Did regular backups and lived with it.
Bored one day, I downloaded the crash log analyser tool from the Windows 7 SDK and learnt how to use it. All of the crashes turned out to be wi-fi driver power state errors, and always the same. No new drivers available. Out of support with Samsung. No solution. Tempted by the free Windows 10 upgrade I bit the bullet against Samsung's advice. The problem became much less frequent with new Windows 10 wireless drivers from Microsoft, maybe once in 3 months at most. So I bit the bullet again and decided not to buy a new laptop just yet, but to get a Samsung Evo 850 SSD and while fitting it, to replace the wireless module with a dual band version to get 5GHz band and Bluetooth 4. Found a brand new one very cheap on Ebay. Laptop now boots (and does all the interminable daily software package update version update checks), in 90 seconds, where it used to take 17 minutes. Result. An unexpected bonus result was that the new card automatically installed the different wireless drivers needed, on first boot, and it has never powered suddenly off since. The wireless works fine and faster on 5GHz, and I no longer need the Bluetooth 4 dongle that came with a Fitbit, so freeing up a spare USB port.
With hindsight, I should have done more research on the best wireless card to buy. I had only checked for compatibility and the number of antenna terminals, rather than best performance. I could have bought a wireless-ac version for a little more and achieved even better wi-fi speeds, as my router is ac standard. Not sure the laptop antennae would support ac though. The SSD is the best £130 I have ever spent and gives better overall performance than I'd have got from buying many a new laptop at over £1000.
What about the Window 10 upgrade. Seemed to complete succesfully with no errors detected, but Edge browser would not open. Re-installed it using Powershell, another learning exercise, but no luck. Did a clean install and that fixed the issue. The only minor niggle is that the touchpad double-tap to left-click function occasionally fails to work first time, so I swapped a wi-fi driver problem for a much less serious Elan touchpad driver issue. MS issued a new touchpad driver a few months after I reported it, and it's better but not perfect. The physical left-click button always works first time so just need to change my habit of double tapping.