What mSATA card do I need for this laptop?

OUCH - it’s half height mSATA! Half height comes in much worse capacities (relative to full size mSATA, which is the size of a industry standard WWAN card) so your options are limited here. Asus probably intended it be used for a 16-32GB caching SSD since they used the crappy version.

Unless you have an oddly specific need, stick to using 2.5” SATA in this and consider the mSATA dead end unless you want a 250GB boot SSD at most - and even then it’s a matter of when those stop getting produced and go up in price, at which point the mSATA option is dead. The other issue is half height drives are hard to come by (relative to full height), so drives aren’t as available - for less capacity to make the situation worse.

Case and point:

• 256GB half height (KingSpec)
• 256GB full height (KingSpec)
• 240GB Kingston (full height)

You got burned with this laptop. Even my E6540 used full mSATA since Dell wired it up for both WWAN and mSATA, despite the standard being thoroughly obsolete in 2013 because they didn’t implement M.2 2880 when that was the defacto standard when that laptop was released.

Hopefully you’re okay with spending $116 for a drive with a reduced write lifetime compared to mSATA (which is already worse then 2.5” SATA) AND paying about as much as we did for 256GB drives just 3-4 years ago. Compare that to a 2.5” 512GB SSD (Silicon Power) and you’ll see the problem.