Greetings!
I have a server running 2012 R2 and BE 2014 and the data transmission speed to my HP MSL4048 tape library is abysmally slow. The Library is connected to the server via fibre and the server is connected to the network via 10 GB Twinax to a Cisco Nexus. I've tested the LAN throughput by writing a backup job to disk and that transmission speed was in the 3-5 GB/Min range. When we back up to the tape library, the transmission speed drops to 3 MB/Min. Also, if I look at the properties of the library or tape drives in BE, the connection type is "unknown".
I've worked with reps from both Symantec and HP and while all my firmware and drivers are now updated, I'm still getting slow speeds. It's important to note that this device was in use at another location and shipped here, so we know it can get acceptable throughput.