Can you elaborate on the interface you're using with the external HD? Is it USB 3.x? 10 MB / Sec would either indicate slow backup speed or you're using USB 2.0. If it's another protocol, let us know. Are you using compression and / or encryption? If so, how many CPU Cores do you have on the server? Using 6 threads will compress / encrypt 6 chunks at a time - on a 4 core server that would use all available CPU and cause a CPU bottleneck. I'd consider using no more than # Cores - 1 for # of Threads. Also consider increasing the chunk size to 20 MB and see. You could also copy a large file from Windows to the external HD and see what speeds you are getting as a comparison.

A few more questions:

Can you confirm you are backing the server's local disk(s) to the external HD on the same server?

Can you confirm you are accessing the external HD using a drive letter and not a network share name?

, it may also be a good idea to involve our Support Team - please use Tools > Diagnostics to send them the logs, and refer to this thread in description

The external HD is USB 3.0, however I'm not sure any of the USB ports on my server are 3.0. Compression & encryption aren't enabled. 8 virtual processors are on the server (Can you recommend what thread/chunk size I should use from that?). I can confirm I;m backing up a local disk on the server to the external HD. I can confirm I'm accessing the external HD using a drive letter & not a network share name.

I think the first thing you need to do is confirm the interface to the external HD. This is a virtual server and that also may contribute to slower disk server disk speeds. Please copy a file to the external HD and see what Windows reports on transfer speed. Once we have that info, we can tweak some settings to get close to your max speed.

ok. how big of a file do you want me to copy over?

500-1,000MB should do it. You can copy something larger if you have it and just cancel after the speed settles and you note it.

copying a 3.67GB iso file transfers at about 7-19 MB/s ( 7 being the lowest, 19 being the peak).

Well, I think that seals it. Your server, because of a USB 2 interface or slow disks, is only able to move data to that HD in that range. It's slow, unfortunately. You're getting 10MB/Sec with CloudBerry Backup, so you're in the range already. Tweaks will only help so much. I would try reducing the # of threads as contention reading the data can slow this things down. Maybe try 3 and see if that helps. Also, unless you're certain compression won't help, you could turn it on to reduce IO to the HD. If you try it that way, then I'd use a higher thread count like you're already using. Do some trial and error and report back what you see, but there's a chance it's already running close to top speed.