Downloading a sketch via a USB/Ethernet print server.

I am trying to access a Mega2650 over Ethernet using a USB print server. I have tried several different models all of the type that allows "any" USB peripheral, cameras, thumb drives, scanners, etc.. The latest is an Encore ENNUS1. When sending ordinary ASCII data to the Arduino USB port everything works perfect. Can send characters to the Arduino and receive the appropriate response characters back. terminal program > Win7 > PC > router > print server > Arduino. However if I try to download a sketch using IDE 1.5.5-r2 I get the following error message:

Sketch uses 28,498 bytes (11%) of program storage space. Maximum is 258,048 bytes. Global variables use 2,196 bytes (26%) of dynamic memory, leaving 5,996 bytes for local variables. Maximum is 8,192 bytes.

avrdude: stk500v2_recv(): checksum error avrdude: stk500v2_recv(): checksum error avrdude: verification error, first mismatch at byte 0x24db 0x92 != 0xff avrdude: verification error; content mismatch

There is no problem downloading the same sketch via the USB port. Is this a problem with the IDE or with the arduino configuration? What can I do?

For radio EMI reasons I want to separate the Ethernet converter from the Mega board so don't want to use an Ethernet shield.