How to sync NAS and USB drive

All frontview backup jobs create a mirror of the share contents - incremental updates the changed files, full backup copies everything.

Actually, they don't. They can make a full backup first time, then I can only force them to delete files on a backup destaination before making a full backup again. This way I'll have a mirror of the NAS share, but every time hundreds of GBs has to be copied from NAS to usb drive. I want to avoid this time-consuming operation and copy only new files (can be done) AND remove deleted files on source - and I can't see any way of doing that.
There's a relevant option on the Advanced tab in a backup job settings but it's grey out. No matter if I choose share: USB_HDD_1 or usb:External Storage (USB Front) as a destination, the 'Remove deleted files on source' in inactive.
Seems the only way to keep the NAS and a backup on an usb drive in sync is to make a full backup job.

You can use rsync even with a USB drive (using 127.0.0.1 as the destination IP address). Then the "remove deleted files on source" option will be enabled.

Ok. so I'm (probably) half way through. I chose in Destination tab:
1. Type: remote
2. Name: remote:Rsync Server
3. Host: 127.0.0.1
and tested connecton - it worked. But the problem now is that I want to backup a share TEST into a folder TEST on a USB drive (in general, every share on NAS will have its own folder on the drive where it'll be backuped into).
When I enter in a path field:
1. 'TEST', I get a message 'Successfully connected to 127.0.0.1::TEST' but when I start a job I get 'Error backup job 'TEST': error in data stream. Please check remote rsync server settings' .
2. 'TEST/' or 'TEST\' give the same
3. '\TEST' and '/TEST' result in 'Unable to connect to host' message.
What should I enter into the path field to have the TEST share backuped into the TEST folder on the USB drive?

Well, TEST won't work, since that just makes the destination the same as the source.

I'm not certain here, but I believe you need the USB volume name as part of the path. Something like USB_HDD_1_1\TEST

Case is of course significant.

Strange thing, I remember answering an hour or two after StephenB's post but my post's disapeared somehow.
In case somebody had a similar problem: the correct path is "USB_HDD_1/" (w/o quotes). So the final settings are:
Source
type: share
name: share: TEST
Destination
type: remote
name: remote: Rsync server
host: 127.0.0.1
path: USB_HDD_1/
Advanced
Remove deleted files on source: ticked