So following advices for BPi M1 is a bad idea with M3 especially when it's about that what has been asked before (I know Brian, you don't read that much, especially the questions you try to answer).
- BPi M1: slow SD card but real/native very fast SATA (40/200 MByte/sec sequential speeds, random IO as fast as the used SSD due to native command queueing since we're talking about native SATA)
- BPi M3: slow SD card, no SATA at all, okish eMMC and an ultra slow USB-to-SATA bridge (15/30 MB/s sequential speeds, random IO horribly slow since GL830 used here doesn't implement UASP and kernel doesn't also).
Combining BPi M1 or M3 with an SSD and putting the rootfs on it:
- M1: GOOD idea, your gains in speed will be amazing!
- M3: VERY VERY BAD idea since the SSD can not perform at all if used behind the slow GL830 USB-to-SATA bridge. Since onboard eMMC is way faster always put the rootfs on eMMC if you were already unlucky enough to be fooled by Team BPi and you thought M3 or any other BPi would be compatible to M1 in any way (they all aren't)
So the only nice/civil answer to the original questions is simply: Don't be a fool and refrain from connecting a fast SSD to the slowest SATA port in the market. The whole idea is shit but unfortunately the whole 'Team BPi' business model is based on exactly that: Creating the impression BPi M2, M2+ or M3 would be in any way compatible to M1. Which they are not and that applies to hardware, software and especially support situation (since there's no community around M3 and its SoC)