你用sudo dde-file-manager开文件管理器后能打开该文件吗？

maybe if you reboot you would have to chown again?

I didn't need to chown anything after reboot. The owner and group remains qemu-kvm.

I don't know Chinese.

Sorry.
You may try "sudo dde-file-manager" and see if you can access the file in that mounted partition.

I can access everything with this.

just sudo -s?

Yes, the directory is accessible with sudo -s.
In the meantime I've managed to find the solution: i had to launch virt-manager as root (via gksudo virt-manager) and choose a disk image from that directory. The program complained that it doesn't have permissions to access that directory and asked wether i want to fix it or not. I choose 'Yes' and it has created the VM. After making sure that the VM is working, I closed it, started it as a normal user and now it's working. I've used it before and it was always asked wether i want it to fix permission issues or not while running it as single user, so I didn't tried to run it as root earlier.

Deepin seems to have this kind of issue, the last time I tried to access isos from my hard drive(using deepin on usb), I encounter a similar issue. Maybe you can manual chmod the entire drive or alter the vm process group(adding to root?). Not sure if will persist after reboot though.

Were the files on the HDD owned by the same user and group on the USB as on the HDD?

Anyway, i managed to solve it as i wrote previously.