Ultrium DT3 – Amber warning light and Red 3 on LED

Its the PSU light. Usually means one of the several Power Supply Units got disconnected. Check the backside of the server, perhaps a power cable went loose.

If it started flashing on both servers, check the sockets/PDU/plugs that are shared between the two servers, perhaps you have a failure in your power equipment.

As an asdide, note that 100MB/min is far below the minimum speed for tape streaming with LTO 3, so you're probably losing a fair amount of capacity with the tape stopping and starting (ie you're probably getting better than 1.5:1 compression, but this is lost in gaps in the data on the tape). This will probably be rather worse with LTO 4, as I think the minimum speed has gone up.

Disk - Disk - Tape will help with the minimum speed problem, and will give you some capacity for free.

If you're not doing it, strongly consider some kind of scheduled defrag of the disks on the servers you're backing up. 1000 MB/min is not a great level of throughput for gig ethernet on reasonably modern hardware. I'd expect that on even 2 year old machines you should be able to get 1800MB/min (that's only reading from the server disk at 30MB/sec), so there's scope for improvement.

Edit: For LTO 3, you really want a 256KB block size for best performance.

WRT shoe shining, There's no time for the tape to rewind if the buffer runs empty briefly, so it'll leave a gap on the tape.