There are several topics discussing how to backup VM files via USB devices. But that's not an ideal solution as of now. USB device is not supported to be created as datastore mount to host and there's limitation that you need to format it to FAT16 to be identified by host. Then another limitation of 2GB file size set by the file system. Anyway, currently, if you'd like to back up your VM, you may try to mount a NFS datastore and scp the files to it would be much easier given in standalone host mode.
Hello Abid and welcome to the forums,
here is a good overview of backup solutions for free ESXi: https://www.virten.net/2016/04/backup-solutions-for-free-esxi/
GhettoVCB is probably the most popular solution: GitHub - lamw/ghettoVCB: ghettoVCB
Regarding USB disk mounts under ESXi please refer to this blog post: https://www.virten.net/2016/11/usb-devices-as-vmfs-datastore-in-vsphere-esxi-6-5/