Because the root file system (the eMMC drive) is not quiesced. Files on it are changing in a manner that is uncoordinated with the process that is copying files onto the external drive. You will probably get lucky anyway but …
The right way to do this is either
- to use a root file system type that supports snapshots (may or may not be possible currently but not the case for me), or
- to boot from another disk so that the eMMC drive is not mounted (not currently possible? although you can probably go close if you are an extra advanced user), or
- to use JumpDrive to expose the local drives via USB (and hence do the backup on your PC, maybe the only option currently).
Maybe I overlooked some options. Let’s see if anyone jumps in.