If you want something faster than an external hard drive, and better than a USB Flash drive, you can build an external SSD drive with an SSD and an enclosure. I built a 256GB USB 3.1 external SSD for less than $70 that works great on my Mac. Learn how to build your own.
If something is wrong with your Mac, either you or the tech repairing your Mac will need to access the special diagnostics mode. You can do this by restarting your Mac and holding the D key. The results could give you some insight into the problem. See https://support.apple.com/en-us/HT202731 for details.