I bought a CP2102 USB - TTL board from eBay. I added the recommended cap from the DTR pin, cut the RST trace, and attached the other end of the cap to the RST pin. I plugged it into my Mini Pro clone, swapping the TX and RX lines as one would think was logical. I got no joy. The arduino IDE would constantly say, either, avrdude: stk500_getsync(): not in sync: resp=0x00 , while in Windows, or, avrdude: stk500_recv(): programmer is not responding , while in Ubuntu.
As many of you have found, lots of people have this issue. There don't seem to be a lot of fixes. I can't promise my experience will work for you, but, hey, it is worth a try. After scanning a number of forum pages, someone mentioned swapping the TX and RX connections. I figured WTH. It can't hurt. I clicked upload, and, IT WORKED! Woohoo!
tl;dr
If you can't program your arduino while using an external USB-TTL board, try swapping the TX and RX connections.