Hi all, I am facing an error when I tried to mount a USB(let me call it USB_2) device.
I extracted the image which is downloaded from official site, and I wrote the "openwrt-21.02.1-x86-64-generic-ext4-combined.img" into a USB(let me call it USB_1) and I boot from it on my X86 PC successfully (model: HP T520).
Sorry if I'm misunderstanding, but your goal is to write the openwrt-21.02.1-x86-64-generic-ext4-combined.img to your T520's SSD--is that correct?
If so, what I do is grab a lightweight "live" USB distro like Gparted (https://gparted.org/download.php). Write the distro (you can use Rufus or balenaEtcher if you like) you choose to your USB disk (likely FAT32 formatted). Then before you boot from it, copy the openwrt-21.02.1-x86-64-generic-ext4-combined.img file directly to the root of that same USB with the live distro on it.
After the copy completes, boot from the live USB you just created and launch a shell. Assuming /dev/sda is your SSD and /dev/sdb is your USB you booted from, run the following:
sudo su -
mkdir -p /mnt/usb
mount /dev/sdb1 /mnt/usb
dd if=/mnt/usb/openwrt-21.02.1-x86-64-generic-ext4-combined.img of=/dev/sda bs=4M; sync;
Remove the USB and you should be booting into OpenWrt from your SSD after the reboot.
The mount command by default has all the filesystem drivers probe the partition until one recognizes it is is its type and mounts it. That error message is just a warning from the f2fs kmod that during the process a f2fs filesystem was not found. Which is expected behavior if the partition is formatted with a different filesystem. The other drivers silently ignore finding a different filesystem.
If you aren't going to be using any f2fs partitions at all, you could remove kmod-fs-f2fs. Note that the squashfs images for x86, Raspberry Pi, and a few other platforms use f2fs as the overlay, so f2fs must be in the kernel. The optional ext4 image does not use f2fs, but it runs the same kernel with f2fs built into the initial load, so it's going to be there unless you make a custom kernel build.