I basically stole a 6 Month old 2015 4K 21.5" iMac, way too good of a deal to pass up. The downside - it has the 5400RPM spinning HD. I have 2 options: 1) Flip it (I could easily make $300 by reselling) OR 2) Keep it (to replace my 2008 iMac that has an SSD I replaced) and add a Lacie 250GB external Thunderbolt SSD to use as my boot drive. Thoughts ? From what I'm reading TRIM WILL work on an external Thunderbolt SSD ? Thanks !