Where is data corruption during copying files likely to happen?

Assuming both the source and target devices are in good working order, where is corruption most likely to happen (if it does)? For example, suppose that I'm backing up stuff from an inteal HD to an exteal USB drive - are undetected errors likely to occur during read from the platter, inside drive caches/controllers, transfer from the drive, on CPU/Memory, in transit through USB or when physically writing to the target platter? Here I'm conceed with transfer alone, not long-term storage issues like bit rot.

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