I am running a flow graph on multiple E310s using network mode and one of them keeps printing "L" to the screen then stopping. I have searched the internet and found this link: http://files.ettus.com/manual/page_general.html#general_ounotes, but it only explains the overflow/underflow, not what the "L" means. Thanks for the help!
First of all, the network mode was never meant for productive usage – it's primarily a diagnostics tool for manufacturing, and has already disappeared from the development version of UHD, and will probably disappear from the next SD card image release for the E310.
"L" stands for late. This means that you've sent samples to the E310 that have a time spec supplied in the tx_metadata, but that time has already passed when the samples reach the device.