Problems with Samsung SSD 850 EVO in MacBook Pro (mid 2010)

A few days ago I attempted to swap my HDD in my MacBook Pro (mid 2010) for a Samsung SSD 850 EVO. After trying several times trying to install Yosemite (both with the SSD inside the mac, and externally) after prepping the SSD using Disk utility, I eventually manage to do a clean install of OS X Mavericks via USB, with the SSD connected via SATA/USB-connection.

After placing the SSD back into the Mac it would start up and look fine, then start acting up, slowing down, volume control not working, freezing up; generally not working as it should. I ran a disk verification, and it showed up one of a string of error messages, eg. invalid sibling links, "the volume was found corrupt and needs to be repaired", incorrect number of extended attributes, volume bitmap needs minor repair for orphaned blocks, amongst other things..! These messages don´t all come from the same verification. I repaired the disk, restarted the machine, and the same thing happened over again. I had the SSD out of the machine, put the old HDD back in, and connected the SSD via SATA/USB-link; even got it set as startup disk and worked fine!

What is going on? The HDD works fine when back in the Mac, the SSD works fine when outside the Mac, but trying to put the SSD in the HDDs place, everything goes bananas. Has anybody else encountered anything similar, or does anyone have an idea of whats going on? I´ve removed the SSD and put the HDD back. Most probably returning the SSD to the shop where I got it (great return-policy!).

I´ve read that puting in a new SATA-connection in the machine might work, but urely that can´t be the problem when the HDD works with it?

Any help/explenation is greatly welcomed!