Rsync idel

USB is fickle. Avoid if possible!!!

When you connect directly perhaps UASP in the USB interface work correctly. When you connect via a USB hub it might not work, and cause degraded performance.

A powered UASP USB 3 case might be better than a USB hub.

I only use UASP USB 3 cases directly to my RPi4. Either 2.5" SATA SSD in a UASP USB 3 case or a powered dual 3.5" UASP USB 3 box. Performance is OK, but not amazing using rsync.

Depending on settings rsync may do a lot more than just copying files. For instance with default settings it first copies the file to a temp file, then afterwards moves the file to the right place and right name. This means a lot of extra I/O, especially if you have a lot of small files. Also sometimes rsync does crc checks and so on. And if you don't have noatime on the filesystem rsync may access a lot of files and cause extra I/O even if no files need copying.

(Use --inplace to avoid creating a temp file. But that may increase fragmentation and also disables writing of partial files if only a small part of a file has changed.)

Don't use a hub. Connect the HDD to the NAS with the least amount of stuff between. Just one UASP USB interface. I suspect that every time you add one more layer you add more latency and slow things down.

The whole point of using rsync is that backups/snapshots after the very first will only have to handle changes. If you want to backup 12TB using rsync you need to transfer 12TB only the first time. After that rsync only need to check for changes since last backup and only need do what is needed to update the backup to match those changes. Transfer new/modified files. Delete files. This update can be many thousands times faster than doing a new full backup.

For instance if I want to update a snapshot of a 12 TB HDD, rsync only has to hardlink the old unchanged files and copy over the changes. That may take seconds or minutes. But the first transfer might take hours or even days, depending on what is between the two HDDs.

To save a LOT of time, the two HDDs may be in the same PC the first time, both connected to the same computer using only SATA cables. Then the first transfer is really FAST.

After that the two HDDs might be separated far between each other using SATA interfaces, network interfaces, switches, routers and perhaps even USB/SATA bridges and USB hubs. Possibly even encryption and decryption software and a VPN internet connection. And then the transfer speed will be much, much slower.

But thanks to rsync only changes need to be transferred. So it may only take minutes, or even seconds, to create a new full snapshot backup copy of the 12TB HDD at the destination.

This is how I use rsync!

I typically only use rsync to create updated and timestamped full snapshots of my 16 or 12 TB NASes in minutes over a GbE network. I use cron and scripts that handle rsync to automatically create/update the timestamped snapshot copies, and it even purges old snapshots for me.