I've been looking to update and was going to go to an internal SSD, but a very good open box deal has come up locally that's hard to pass up - $1900 for a MRR12LL/A - 3.7 GHz Intel Core i5 Six-Core 8/2TBFUS Radeon Pro 580X. From what I've read, the X5, if made bootable, via TB3 should be very close or identical to the speed of a factory internal SSD. Yes? I care less about boot time speed as I always leave the Mac on 24/7. It's all about the working R/W speeds. (current 2015 iMac Blackmagic test = 600W/1670R on a 2TBFUS) The question of what size comes up. It seems to me that beyond the system itself, most files do not have to be on the actual boot drive. Would the 500GB cover it? I believe so, but wanted to confirm. I'm trying to make the next box as lean and efficient as possible to eliminate any conflicts or slowdowns. I currently have ~100GB of programs in my Applications folder (many of which aren't used) - but most programs do not even have to reside on the boot volume, right? The plan on the next iMac is to add back only the programs I use as needed and archive the rest. For massive file storage and easy regular access I've already relocated Movies/Pictures/Music/Documents to redundant externals (synchronized via Chronosync which I also use to maintain an extra bootable backup - in addition to my TM external running - yes, redundant backups too!). Back to the X5 - if booting from one can make a big difference in working R/W speeds (particularly Photoshop/Lightroom) over the stock internal Fusion drive - it will be well worth adding. Of course I would keep the internal fusion drive as the backup/emergency - and just use the rest of that 2TB as one of my redundant Chronosync archive folder drives. I should also be able to use Chronosync to keep the system and user settings exactly the same from the external SSD to the internal as well (since it supports bootable backups and mirror copying). Any feedback would be appreciated!