Not sure if the Pi HAT with mSATA has an advantage over just an SSD, if so please share, but here is what I use and no issues thus far.
I read many articles on moving a setup from one device to another and thought I would give it a go. For anyone interested in what hardware was use (seem in most forums this can be an issue) RPI3 B+ with 5V 2.5A power supply, PNY CS900 SSD 120GB (bestbuy on sale $25) and a USB to SATA cable (amazon $12). Here is a link to the cable https://www.amazon.com/gp/product/B00HJZJI84/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1 lots of issues with using RPI3 with no SD card seemed to come from cable compatibility. I’m waiting on a 2.5" hard drive enclosure w/ USB cable connection, if it works I will edit and include a link.
I changed from using the cable above to this enclosure and have since bought 2 more of these.
Hard drive case came this morning, sooner than I thought, and it works great! I had no issues with the USB to SATA cable, it seemed to respond quicker than before (Note: I was on a RPI2 before). After shutting down, swapping connections, then restarting I can definitely see a difference in response. I know I’m limited to USB2.0 speed due to the RPI3 but I’m pleased with my new setup. Here’s the hard drive case w/cable info for anyone thinking about upgrading. (amazon $8.59)