I think none of the mSATA solutions work with Rockbox right now, only original Apple firmware. Not sure how long it's gonna be until the Rockbox devs figure it out so I just ordered a Toshiba 160GB to upgrade my 5G 30GB.
Tried the mSATA adapter and none of the iPods like it, on the iPod 6th generation all I got was a red X and on the 5.5th it showed a sad face and then turned off.
Tried it using both a 24gb mSATA card and a 256gb mSATA with the same results.
I have available the adapter and 24gb card I can mail it to anyone that would like wants to try them on his iPod.
Edit - Tried the 24gb mSATA card with the 5.5th gen iPod and this adapter the iPod accepted it, connected it to iTunes and restored then after done and working fine I went ahead and did the RockBox installation, the iPod rebooted and got this same error on the screen like other guys are getting with the 256gb card. So this means that the card size have nothing to do with the error, maybe the mSATA to zif adapter is the problem here?