Yes, create a datastore on the usb drive and set a local sync from source DS to the USB drive. Remember to set datastore offline before unplugging the USB drive.
Wait for all running tasks on that datastore to finish, from webUI set datastore to offline maintenance mode [1], then wait like one minute for proxmox-backup-server service to close all files in the datastore, unmount the drive and then you safely remove the drive. If in doubt, check with lsof | grep <datastore_mount_point> if there's any open file before unmounting.
That makes me dizzy. I'd rather stick with my external script solution:
1) mount an external USB disk on the PBS.
2) use a script to create vzdump files on this shared disk from each client.
3) unmount the USB disk on the PBS.
4) live with the fact that data is sent twice over the network unnecessarily.
5) be happy that data is available without a possibly defective PBS.
You asked for a solution without a script but you use a script... Of course, having both types of backups (PBS and vzdump) add redundancy and data safety. If you prefer that, it's a perfect choice. Mine would be to use a script that gets invoked via udev when a given drive is inserted in the PBS host, syncs the datastore and once done sets the usb datastore as offline and sends me a mail to tell me that I can remove the disk.
I thought I had recently read something about USB disks instead of backup tapes. It was probably just a hallucination and I'm sticking with the better parallel script solution.
Thats the functionality needed for all this to work . Previously, even if you set a datastore offline, PBS service still kept some open files in it and you could not unmount it unless you stopped PBS services, which at the very least is a PITA and in most cases simply not possible as that stops any running task.
@TErxleben maybe you could open a feature request in bugzilla [1] so devs know there's some interest to get this done natively in PBS.