Well fizze, you described my setup perfectly, usb ext stick and ntfs hdd.
I know ntfs is not recommended, but nor fat or ext where an option. But I did not read anything specific preventing such a setup to freeze (it was more in terms of days in my case).
Anyway, got rid of the flash and caved in to move to ext hdd 😳
Lost the goal I wanted to achieve of being able to take the hdd at someone else’s place and just plug it in and transfer 🙁 But gain stability …