Doesn't fit, period.
First off, it doesn't seem to be an issue of the height pushing the caddy up and the bottom of the case not fitting... It's the height pushing the caddy up and not allowing the connectors to line up.
Follow me? On some laptops, there is a ribbon cable that connects the motherboard to the drive via an adaptor that usually mounts on the drive or caddy.
In our case, the connector is hard mounted on the motherboard, and since the drive is ever so slightly taller than the caddy, it causes the connectors to just barely not line up.
In other words, if the M11x had a ribbon cable, the 12.5mm drives MAY
fit. I didn't even bother trying to put the bottom back on since there was no way of getting the connections going, so there's a chance the bottom wouldn't fit on correctly either...
Alright, I promised pics...
The original hard drive installed:Attachment 19986
The original hard drive in the caddy... notice there is a tiny amount of "expansion" room above the drive... maybe it's for airflow?Attachment 19987
Hard to see, but the 12.5mm drive causes the connectors to just barely miss each other:Attachment 19988
The angle where they finally meet up! :PAttachment 19989
The 12.5mm drive in the caddy:Attachment 19990
Just for fun, tried in the wife's Studio 1535... her 9.5mm drive installed, showing lots of room for expansion AND a ribbon cable!Attachment 19991
BUT it doesn't fit
Turns out the shape of the Studio 15 case causes the drive bay to be wedge shaped, with the smallest height being 9.5mm (tallest height is over 12.5mm)... so this happens when you try to put the bottom back on:Attachment 19992So yeah... moral of the story is:M11x ≠ 12.5mm
Newegg was having a deal today on the 9.5mm WD Scorpio Blue (same family as the 1TB, but with less platters) 640GB... $84.99 free shipping, so I purchased that one and will now go try to attempt to find a 2.5 enclosure that fits 12.5mm or a 3.5 enclosure and maybe get one of those SSD adaptors to throw it in.
In any case... just buy the 1TB WD external since it's the same drive, just $40 cheaper... and NO, you can't buy the external and break it open and take the drive out... there are no SATA connectors on it... the controller board is only USB.