Hi Cgiles,
Please refer to information below , hopefully could be of some help:
1) Please try disable the power management option in the property of I218LM in device manager
2) Regarding the transferring of the file of several hundred MB or a couple GB then it'll copy about 10 MB then disconnect, does this same issue occur if you tried to transfer file lower than 100 MB, like if it will copy about 10 MB then disconnect?
3) What was the driver version for I218LM before that you encountered the SEP and sleep issue? Is the SEP version 12.1.5 as well?
4) Have you contacted Symantec support about this issue?
rgds,
wb