Recapping…
- Raspberry Pi 3 Model B+
- External hard drive
Like its predecessor, the RPi 3 B+ has a single USB 2.0 bus. Although the RJ-45 port supports gigabit Ethernet, it’s capped at 300 Mbit/s. Another caveat is that the RJ-45 port is actually an embedded USB-to-Ethernet adapter, so it shares the same USB bus as the 4 onboard USB ports.
Broadcom’s USB chip is also a hybrid combination of hardware plus software (i.e., portions of the USB functions are implemented in the Linux driver). USB 2.0 in theory maxes out at 480 Mbit/s, but actual speeds will vary significantly depending on CPU load, how many USB ports are in use, wired or wireless network connection, etc.
Okay, so max transfer speed is 50Mbit/s = 6.25MB/s (based on the uplink speed on the MacBook side) before overhead and latency are factored in.
That works out to just over 1 hour per 25GB under perfect conditions, but realistically 50% or less of that at best.