I plan on buying an Asus Zenbook UX303LN. It has a 256GB M.2 SSD plugged into a M.2-to-SATA card, and I suspect Asus probably had a surplus of M.2 SSDs. The laptop does have a spare M.2 port, but it requires a half-size card, and those are significantly more expensive than the full size M.2 or 2.5" SATA SSDs.
This is a picture from a review website:
Based on the label, it looks like it's a Sandisk x110 265GB M.2.
The spare 2.5" SATA drive I have is a Crucial M500 240GB.
I intend on reformatting the laptop and installing a fresh OS, but I can't decide if I should leave the M.2 SSD and the M.2-to-SATA circuit board in the laptop, or replace it with the 2.5" SATA SSD.