I had what would seem to be a similar problem, running 12.0.  Same basic setup and Backup size, but the backups are being written to a large RAID5 array.  Backups would start out fine and then slow down to about 270 Mb/min.  This started all of a sudden, previous speeds were around 3500 Mb/min, then one day it stopped working.  I discovered that the write cache on the storage array that backup exec was writing to had been turned off by the controller when i swapped a HD out and it rebuilt the array.  It never turned the write cache back on when the array returned to normal state.  I manually re-enabled it and the speed immediately returned to normal. 

May not be applicable to your situation but you might look into the caching on those external drives/controller.

Best,
joe

Just to ask a stupid question - what's the nature of the source backup?  Is it possible that the backups start with large files, then toward the end run into many small files?  If this is the case, you're running into the overhead to open & close each of those files - which there's no easy way around.

You should investigate all possible disk tuning settings both in the controller, the partitions, the formatting options, etc.  

This is good practice in general.   Not to say that BE isn't at fault yet, but troubleshooting the simple stuff that you have good knowledge of before diving into an app is often at least for me, easier.