To paraphrase the cop addressing Jack Nicholson’s character at the end of Chinatown: 
‘Forget it Jake, it’s the internet.’

Exactly where does the iconic and eminently Instagramable majesty of Sahara start in Morocco? Like Keyser Söze, is it even there at all?
Years ago I came across an internet forum argument on the now defunct LP Thorn Tree (I gave my 2ç then left them to it). There, a couple of know-alls lambasted the callow innocents daring to enquire where they could ‘see the Sahara in Morocco’ – in most cases referring to the famous Erg Chebbi dunes. ‘I told you: there is no Sahara in Morocco!’. All concerned have now migrated to TripAdvisor from where the quotes below were copied. It staggers me that some of these individuals can assert their unsubstantiated beliefs with such conviction, but such are the times. According to one of TA’s wise owls, ‘Erg Chebbi is not in Sahara, desert it is but not Sahara‘. So the Sahara is ringed by an unnamed desert that is not the Sahara?

… neither … Erg Chebbi/Erg Chigaga are the Sahara. For that you need to head to Western Sahara.
Ok, we know that but we want just see Sahara and ride camel
The limit of the Sahara is classified by the limit of the Date Palms
the Sahara,… lies 100 km or more to the south in Algeria.
Although not part of the Sahara , the dunes are pretty amazing.
It also corresponds to the 100mm isohyet in the north and the 150mm isohyet in the south. Long term average rainfall. A further definition is where the evaporation potential exceeds the presipitation [sic]. 
I don’t care what “Sahara” means. I am talking about the actual geographical boundaries of the area. And your definition of those boundaries is as incorrect and misleading as that of the ridiculous tour companies’ one.
Erg Chebbi is not in Sahara, desert it is but not Sahara. If you by reading see on website about Sahara tour, then it is only marketing to attract customers. But Sahara begin first in Algeria or you have to go to the South of Guelmim, to Western Sahara.
Here we go again with the “Erg (Chebbi/Chiggaga) is not the Sahara” brigade. What’s wrong with you? What’s a few hundred miles between friends? Jesus H. Christ.
May I just make one thing clear? You will not be travelling through the Sahara desert until you have gone south of La’ayoune. References to the Sahara in other places such as Erg Chebbi and so on are simply concoctions dreamt up by tour companies to fool their victims into thinking that they have been to the Sahara – they haven’t. To the east, the Sahara starts way over the other side of the border, deep into Algeria.
the Sahara if [sic] 100’s of kilometers away from the big dune areas over the border in Algeria and not in Morocco [continues] ... [Chebbi] is more spectacular than any of the places in the Moroccan Sahara that I have visited.

This knotty problem of the Sahara’s precise extent may be unique to Morocco. You start with the very concept’s compelling mystique. It’s hard to think of other wilderness regions that conjure up such strong imagery and notions, maybe because it’s so close to Europe compared to other iconic wastelands.
Then you mix in the unceasing clash between gullible bucket listers wanting to definitively tick off the Sahara – and devious local tour ops who promise the earth and deliver a turd. One wily tour agency has even managed to insert themself as a ‘reference’ on Wiki’s ‘Sahara desert (ecoregion)’ page.
This seems to be the angled grinder which the naysayers above fixate on: if some Moroccan cat in an oversized cheche strolls up and says ‘Hello my friend. Come, we will drink tea with nomads and I will show you Sahara’, you can expect to be a little disappointed.

As with all geographical features (seas, mountains, etc), it’s tempting but futile to apply fixed boundaries to their precise extent, as if they were a country. Where exactly does the Southern Ocean become the Atlantic? Where does the Karakoram become the Himalaya, or the High Atlas become the Anti Atlas for that matter (along the N10 highway, of course!).

It’s the same with hot deserts, commonly defined as arid regions with less than 100mm of average annual precipitation. Unlike a sea shore, there can be no finite edges identifying this change in rainfall; to imply otherwise would be absurd. In this way the NOAA map (above) is rather more nuanced, showing blurred transitions, not fixed isohyets (rainfall contours) between regions. Both the hard-edged yellow and red maps below could benefit from the same idea.

According to this ridiculous website, (sadly, the first that popped up on my Google) it appears to rain every other day in Merzouga/Erg Chebbi. (The ‘sea temperature’ there gets pretty balmy too – or maybe they mean ‘sand sea’).
This source for Merzouga looks rather more plausible: 65mm. Another estimates 53mm. yet another 172mm. This French Wiki gives 59mm. Let’s assume it’s not Kew Gardens down there and it comes in under a 100 mil most years.

Not all accept the level of aridity as a definition (‘I don’t care what “Sahara” means…‘), but they don’t cite any sources to explain their assertions about the Sahara’s boundaries. Instead, some of the TA dolts avow the Sahara lies ‘100km south of Erg Chebbi’ (ie: in Algeria). Others insist ‘200 miles’ from Chebbi. Final answer: ‘100’s of kilometers away’. Some even proclaim they magically entered the Sahara at Guelmim in western Morocco because ‘it is officially known as ‘The Gateway to the Sahara’’, surely a bald, tourist-oriented claim in a town famously thick with faux Tuaregs.

Others insist that, to be in the Sahara in Morocco, you must go ‘beyond Layounne’ (left), a fairly boring road trip, even for a dedicated bucketeer. What will they see on the way there? It may look like a desert, walk like a desert and quack like a desert, but it sure won’t be the Sahara!
No matter – give it a few decades because it’s said the climate in Morocco will see the Sahara creep over the berm and into Morocco.

You can see why some of your forum know-alls get cynical, myself included, even if I believe they’re dying on the wrong hill on this one. Lately, some canny marketing has designated an area of eroded wasteland ever so conveniently close to Marrakech’s gated resorts as ‘the Agafay Desert’ or ‘le petit Sahara’ which ‘extends over several hundred acres’.
There you can do all your favourite deserty things: sip tea in a tent, wear a cheche or ride camels, quads and 4x4s across a landscape where the barren rounded hillocks of dried mud resemble dunes. Insta heaven!

Some academics have sought to reclassify the liminal sweep in question as the ‘North Saharan steppe and woodlands‘. Aka: the edge of the Sahara where things gradually become less arid, like the Sahel in the south. But looking at the zone on the map below, ‘woodland’ is not a word which springs to mind. The band encompasses the vast Grand Erg Oriental spreading across Algeria (left) and southern Tunisia – about as Saharan an expanse anyone could wish for which would swallow up hundreds of Chebbis. The only woodland here is of the petrified fossil variety, west of Hassi bel Guebbour.

Me? Having travelled much of the Sahara over the last five decades, if you pushed me blindfolded out the back of a Hilux behind Erg Chebbi and removed the hood, I’d get up, smell the air, squint at the sun and guess I was in the Sahara. It may transpire to be the edge of the Sahara in boring old Marrokie, because vegetation and rainfall change in a very short distance (by Saharan standards) in the mountains to north.
But from Chebbi south and west along the Algerian border all the way to Tan Tan, it all resembles the Sahara I’ve seen – often dreary, sometimes epic, often barren, vegetated in patches, hyper arid elsewhere – all the way from Mauritania’s Adrar plateau to the Selima Sand Sheet (left) close to the Nile.

If anything, Erg Chebbi is just too darn Saharan for school and so has become one trampled terminus of Morocco’s Axis of Tourism, a quad-busting desert resort. I avoid the place. Over the years the pretty cluster of dunes at the end of the road has become choked with desert camps, kasbah-hotels, a few opportunist sharks and us tourists doing the dunes by various means.
In that sense it is very much not what I know of the real Sahara, but caught right, Chebbi can still be an amazing sight for a desert first timer. You’re on the very edge of the Sahara. Which side you’re on is up to you.