I thought I'd take a closer look at what was happening with my netcat problem.
I ran a cmp on the files to see just what was happening.
cmp -l /media/sdk1/BACKUP_2018_01_15_BY_USB /media/sdk1/BACKUP_2018_01_15_BY_NETWORK 10056230615 5 0 10056230616 152 25 10056230617 264 351 10056230618 151 57 10056230619 14 205 10056230620 357 125 10056230621 156 0 10056230622 326 32 ...
(Oh and if those "bytes" look weird, cmp outputs the bytes in octal...go figure. cmp prints the byte offset in decimal and the byte itself in octal)
and what's weird is that the error blocks are 72 bytes in length. (I think that's the length of an ethernet frame)
and even more strange is that the error blocks are always the same exact data. Above you can see the sequence 0,25,351,57 etc...
That's the same 72 byte sequence in each error block.
I loaded the output into LibreOffice Calc and graphed it:
So out of 250GB of data, there are around 432 bytes different, 6 blocks of 72 (72*6=432).
Column H and I are just =CHAR(OCT2DEC(B1)) and = CHAR(OCT2DEC(C1)) copied downward.
I actually did the copy over the network twice and got different results:
(edit: I thought it was a realtek 8169 but it's actually a Marvell Yukon)
The sending network card is a Marvell Yukon (D-Link DGE-530T) using the skge driver running under Knoppix 8.1.