Took the SSD that used to be in the 2011 MBP, the one that died recently, reformatted it & installed El Capitan on it & put it into the 2009 13-in MBP I've been fixing up for the Mac User Group. Just for research purposes, though. I wanted to see if it would be a reasonable performed even with only 4 GB of RAM installed.
It does quite well, actually, the combination would be very workable for a light-duty machine.
The HDD is back in it now.