It's about 10 hours and it's still on the formatting screen. I haven't been able to see if Rockbox works. I can't remember what was on Beyondwind's website. Was there any information regarding emcore?
I tried using stock iPod OS; whilst syncing with itunes it freezes randomly. Once at track #250 out of 9850 and then at #2560 out of 9850. I have to "unsafely" remove the USB cable which causes the iPod to restart, and then I'm greeted with a red cross.
It was definitely not formatting, I can tell you that. Should take less than a minute. Beyondwind did provide a tutorial of sorts, but you can find the emCore install instructions on their own site: http://www.freemyipod.org/wiki/EmCORE_Installation
Ironically, I'm still planning to revert to stock OS myself. Copying tracks with Rockbox rendered my iPod useless multiple times. FYI I'm using an 840 EVO as well.
According to TheSeven (from emCore) and Tarkan, the problem might have to do something with the SSD entering 'offline mode' when transferring a large batch of files. What I remember is that modern SSDs have a certain caching mechanism that iPods don't use. When those caches stay empty for a while, offline mode is triggered. This might explain your syncing trouble.
The difference between emCore and stock OS, though, is that the emCore team didn't have any technical sheet to work with, so to say, and the Apple team did. Both Tarkan and TheSeven recommended I revert to stock OS because it's just way more stable with mSATA. See http://www.tarkan.info/20141213/tutorials/1tb-ipod-classic-restore-sync-video
In any case my SSD is spitting out errors because of all the misuse, so I have to take it out again.
TLDR: Stock OS > emCore/Rockbox when it comes to mSATA. And don't sync large batches of files.