In June 2012 I have bought Asus Zenbook UX31A (model UX31A-R4003X). It has ADATA XM11 256GB SSD in similar form type as M.2 – Be careful disk inside UX31A is NOT M.2, they are not compatible. Disk is some kind of proprietary form type only found in Zenbooks. Image bellow is showing my ADATA and M.2 disk.

In 2.5 years I outgrow this disk and I had to upgrade to 512GB. Asus’s decision to use proprietary connector gives us a lot of troubles. To solve this issue we have to use an adapter. I bought this one (see edit 11). Here is an image if amazon stops selling it:

Disk I bought is Crucial M550 512GB SATA M.2 Type 2280 (CT512M550SSD4). You have to be careful because not all M.2 drives will work. It has to support SATA data transfer and not only newer and faster PCI-E mode!
To open UX31A you have to unscrew 10 T5 screws on the bottom plate. To change disks, you have to unscrew additionally one philips-head screew that holds disk in place. Here is a photo of disk and adapter fitted inside laptop. Adapter and disk is a bit thicker in comparison to old one. Bottom lid is a bit harder to put on, because of thickness difference.

To be on safe side I insulated the aluminum lid where it comes in contact with the disk with antistatic bag in which electronics usually comes in.
And because last post was on disk speeds, I must show you results of speed test on new ssd disk (left ADATA MX11 256gb, right CRUCIAL CT512M550SSD4):

As you can see disk speeds were upgraded considerably. We will see how that will transfer to real life.
EDIT: Someone in the comment selection asked for disk speeds of the Transcendent disk. Because CrystalDiskMark changed its test I have redone tests on my CRUCIAL disk, so they can be simply compared. As we can see Crucial has better writing speeds in nearly all the cases, and similar reading speeds.

EDIT: Check the comments section for additional details (how is disk working out after few days of usage) and tips on how to copy/transfer windows.
EDIT2: Commentators Uli and Holger report that Sandisk X300 works too. (Thanks Uli & Holger for the feedback! )
EDIT3: Commentators Lars and Werner report that they used Transcend 512G MTS800 M.2 SSD and it works as expected. Florian also used Transcend’s disk. (Thanks Lars & Werner & Florian for the feedback! ).
EDIT4: Commentator Stefan used Sandisk X300 SSD M.2 2280 256GBSD7SN65-256G-1122 (Thanks Stefan for the feedback! )
EDIT5: Commentator Stephane used same adapter and “Transcend TS256GMTS800 SSD interne 256 Go M.2 (Type 2280) SATA III” disk on older model – UX31E . (Thanks Stephane for the feedback! )
EDIT6: I have succesfully installed Transcend MTS400 M.2 SSD 256GB SATA III, MLC into my GF’s UX31A. This disk short one (42.0mm long, Type 2242) and it fits without a hitch.

EDIT7: Commentator Eric used same adapter and “Samsung EVO 850, 500GB”. He reports some problems with back light and touch pad, we are still trying identify the problems and figure out if they are caused by the new disk (solved: there were wrong screws in wrong holes). Jakob reports that Samsung EVO 850, 500GB works but has ” a little tight fit”. Check below for in the comments. Pete is reporting that Samsung 850 EVO M.2 250GB works as expected. Oliver used Samsung 850 EVO 250 GB M.2 3.5-Inch SSD (MZ-N5E250BW) (Thanks Eric , Jakob, Pete, Oliver for the feedback! )
EDIT8: Alte has found new type of adapter that should minimize problems with thickness. (Thanks Alte!)
EDIT9: Acording to Michael Crucial 500GB MX200 Works as expected (Thanks Michsel!)
EDIT10: Svankius used Adata Premier Pro series ssd (ASP900NS38-256GM-C) (Thanks Svanikus!)
EDIT11: m3mp5 used different type of the adapter it’s smaller and could be better. (Tanks for the feedback m3mp5!)
EDIT12: Thanks to Sarah, please consider this comment when deciding about adapter.
Original disk inside UX31A:

Insulating the lid where it could touch the disk with a simple plastic anti-static bag and some electrical insulation tape:
