This is the most common error. One should connect the external HDD to the computer used to control the data transfer (if possible) otherwise the data gets transferred from the external HDD over the network of the Nucleus to the computer that controls the transfer and back again to the Nucleus and onto the internal SSD, halving the available bandwidth and thus doubling the transfer duration. One should also use a wired connection for the transfer – move either the small Nucleus, the PC/Laptop or both if needed.