Unlikely. There used to be 8TB heliums at some point, but then WD replaced that line with air-filled ones. A process that at least went to the 10TB line too already, I believe. Your safe bet for helium is 14TB and up, I would say.
When you buy that 6TB drive make sure to get the WD60EZAX (aka CMR) and not the WD60EZAZ (aka SMR). It’s one letter difference… if you buy from a random dealer they may send you the SMR.
Typically in the specs sheet of the manufacturer. You have to read/ look closely. Best is to ask about a specific drive ID then we can tell more (out of experience).
If you have backups it doesn’t matter much what you buy. It’s not like there’s much choice anyway. :)
Do you want to buy a small 2.5" one? Or a larger 3.5" one?
All the small ones are SMR these days, so that’s something you should consider. Depending on exact use cases this may become very slow, and in case of error, like a sudden drop of power source which easily happens with externals, they also corrupt more easily (in other places, not just necessarily in the data you just wrote to them).
If you go for a bigger 3.5" drive go for a larger size and CMR. 4 TB is laughable and ultimately more expensive (EUR/$ per TB). I would recommend to look for 12TB and upwards with helium filled. The 18TB disks are the current price sweet spot it seems.
It’s a 20% chance, I guess. Dunno. :) If it has a flat top surface and no air holes then it’s Helium, else it’s air.
That’s a helium one: https://pcper.com/wp-content/uploads/2016/03/d177-dsc00234-1280x640.jpg
That’s an air one: https://cdn.eteknix.com/wp-content/uploads/2019/04/Western-Digital-WD-RED-8TB-Photo-view-angle-1-880x850.jpg