thumb drive for samples?

With some caveats, this is usually fine.

First, data transfer speed depends on the USB protocol , drivers, and mfg., the speed of the processor and the way the DAW handles data transfer to name a few internal issues. Does it load the complete sample into computer memory or is it drawing from the USB in real time?

Second, thumb drives have internal components that are optimized for different uses. The drive you choose should be stable and fast. Thumbs are faster than before, however, the larger drives may have lower seek times, etc. when compared to an external solid state drive.

Third, always, always , always have multiple backups of your sample drive!
If your system has USB 3.0 ports I'd buy a small ("thumb drive" size) M.2 enclosure and the size memory you want. Then you shouldn't have to worry about transfer speed.