Solid State Drives
SSD in external enclosures
Samsung T5
These are nice and small – about 2/3 the size of a spinning disc pocket drive. They are USB 3, not thunderbolt, so they top out at about 540 MB/s. This is a nice option for a backup or working drive, but not as fast as a thunderbolt/NVMe version. 
Samsung T5 come in sizes from 250 GB to 2 TB. I suggest buying the biggest one you can afford.