In ESXi 6.5, there are some changes concerning devices connected with USB. The legacy drivers, including xhci, ehci-hcd, usb-uhci, and usb-storage have been replaced with a single USB driver named vmkusb. The new driver has some implications if you are trying to use USB devices like USB sticks or external hard disks as VMFS formatted datastore.
Some people have reported that they have issues with USB Datastores since ESXi 6.5. I've tried to reproduce and fix those problems. This post explains the changes in the new version and how to create VMFS 5 or VMFS6 formatted USB devices as datastore on your ESXi host.