Nicolai makes some good points, however.
"One of the drives currently has 1.1TB of data on it. A full backup of that drive directly to tape (as I have no other option) takes 49 hours."
A couple of things are not clear :
Are you concerned with only backing up that drive ? That is, the other drives backup quickly, perhaps they are much smaller.
Multiplexing is used to allow more data from multiple disks or even machines to go to a drive, thus allowing the drive to stream. Certainly in this case you need to be getting your drive to write quicker else the shoe-shine effect you will have is damaging, this is dependant on the drive type, some drives are designed to stop-start and suffer no adverse effects.
Mpx.ing will not backup that one drive quicker - the limit is either the speed that data is coming off the drive, the network or the tape drive.
You could run a bpbkar -nocont test to test the speed at which the bpbkar process can read data. What ever this figure is, if, low, is a limiting factor.
Next FTP 100MB from the client to the media server, this is the network performance speed.
Finally, write data from the media server to its own drives, this will probably be very fast (depending on the speed to media server can get the data off the disks.
If I had to guess, I suspect that the llimit is caused by the speed the win 2003 machine can get the data off the disk. Not really that much you can do about that in NBU, apart from using flash, which would increase the speed. The other option is to put the data on striped disks, it might help. However, if there are many many small files, this is a limiting factor with any backup software, and the reason flash is used.
Multiplexing off one drive is bad, as the drive heads are flying all over the place and in fact the backup will be slower.
However, you say a 1TB drive, is this in fact a 1TB lun made up of multiple spindles, in which case as long as each stream is pulling data from a diffrenet spindle, although in the same LUN, multiplexing would help.
Martin