iMazing fails to connect to an external drives or NAS while backing up your device. The macOS's Finder shows multiple drives with the same name and '-1' appended to their names.
This can happen when a drive has not been disconnected properly. In some cases macOS will not unmount the remaining mount point, for instance /Volumes/Data and will instead mount a new mount point with a new name /Volumes/Data-1, etc. If this happens, iMazing will not be able to find the previous mount point and will ask you if you want to "Retry" or "Change Backup Location".
In that case, it's better to not click "Change Backup Location" but instead restart macOS or try to fix the issue by following instructions in these articles:
Recent macOS versions are also less prone to this kind of issue so if you are running an old macOS version, you should try to upgrade to the latest one.
You should also check in System Preferences your Energy Saver options and make sure "Put hard disks in to sleep when possible" is not checked.