General question about cloning Save Sets onto USB drive

@VictorGh , You can definitely do this but its is not recommended to use a USB drive as a backup target, they are not reliable at all. Moreover USB drives are way costlier compared to internal HDD's or a tape drive and tape, so cost wise the decision does not make any sense. Unless you have a USB 3.0/3.1 on you server(which is again very unlikely) the operation speed is going to be pretty low.
But even with all these factors considered if you still want to clone data to it, All you need to do is plugin the device and create an AFTD device on the drive that shows up in the OS. Then you can either do a manual clone from the UI or using the nsrclone command OR use create a clone based workflow.

Apart from the fact this would be very bad practice, to store backup data on an usb disk, without having any protection going on for that single drive (*), NW itself has no actual approach for rotating disk-based media to make that easy.

Better than USB media would be to use a fileshare on a NAS and make that available to the NW server by then creating within Networker an AFTD (advanced file type device), which is nothing more than pointing to a directory on the share and creating a backup volume on that.

But you would have to look into creating further AFTD's as I assume you are now also using AFTD's as you mention running out of diskspace on the NW server? So you would then already know about managing and creating AFTD disk-based backup devices? So by unmounting the atd after it is full, unmounting the usb drive, add another usb drive, then label and mount a new volume with another name on the device, and then you could continue. The previous backups would then be on for example usbvolume1, while now on the 2nd usb drive you would have created and mounted usbvolume1, or whatever naming convention you'd use to name the usb based NW backup volumes.

(*) Then again, it's easy to talk as we have purpose build disk-based deduplication appliances (Dell data domain, DD), so data consistency is being controlled by the DD, which also has raid and a filesystem with self-healing mechanisms. And can leverage Client Direct meaning that using the DDboost protocol from the DD, clients can perform client-side dedupe, so that only the non-dedupe data blocks would have to be send over the network to the DD.

To get the data from the current backup pool to the newly to be created pool on the usb drive, you would be performing so-called staging. Staging is in effect cloning the backup data from one volume to another and once succeeded it will delete the original. The approach depends on what the selection criterium or criteria would be be, to properly select the ssid's to be staged? Using either NW NMC/NWUI to create a protection group of type query and selecting the appropriate items? Or to script it and use nsrstage, possibly with an input file stating the required ssid's?

Wrg to staging have a look at if doing this automatically via a staging policy or in a manual controlled way, only once necessary. When done manually you would not need to use a staging policy, but a clone workflow (with the option ticked to delete the original once cloned) or use the nsrstage command (or the nsrclone command using the -m (migrate) option as weirdly still the nsrclone command has some options that the nsrstage command does not have, for example nsrstage misses the option to designate a save storage node using "-d").

See for example https://www.dell.com/support/manuals/en-us/networker/nw_p_nwadmin/staging-save-sets?guid=guid-aa54b46f-0d9e-4e9e-98de-4f69e6fcc808&lang=en-us

And for cli examples https://www.dell.com/support/manuals/en-us/networker/nw_p_nwadmin/common-networker-staging-commands-and-issues?guid=guid-f65e67ee-9d42-4d1e-8800-993883e397e9&lang=en-us.