When I do a backup in TWRP on my Honor 8 (FRD-L09), it invariably end with a message saying that it failed to unmount /system due to device or resource being busy. The header still says that the backup is complete, and that it was successful.
In case it makes a difference, I'm running FRD-L09C432B131, which is EMUI 4.1, Android 6.0. Kernel is 3.10.94 from 11 October 2016. This is what was installed on the phone when I got it. I got the bootloader unlock code from Huawei, by registering and applying for it. TWRP is 3.0.2-0 downloaded from here and flashed using fastboot on Windows 10.
Here's a photo of TWRP after finishing the backup:
Need I worry about this?