Why does my SQL Server in AWS have problems backing up to second drive?

I'm running SQL Server 2012. This is running on Amazon Web Services and it hosts 3 relatively large databases (from 3GB to about 40GB).

So far, I've been creating backups in the local drive (system drive, C:) without any problems. With problems I mean noticeable negative impact in database server or application performance.

Problems started when I decided to add a second drive to the server (attached a new EBS volume to the AWS instance). Now, when I backup to this new drive (E:) the performance is affected, a lot. Server pretty much stops responding.

I looked at my monitors and it doesn't look like the server is under more stress when backing up to E: than to C:. Disk read/write, which was my first thought, looks pretty much the same. Both drives are SSD EBS volumes.

Maybe I'm not supposed to backup to a second drive? Am I missing something? I looked around but all similar issues I found are about backing up to network share or permissions related, which doesn't apply to my problem as I'm not backing up to a network share and permissions are OK (the backup is done correctly, but at a high performance cost).