I have recently noticed very slow transfer speeds moving/copying data from the cache drive to the unraid array.

We're talking in the order of 10-20MB/s kind of speeds.

 

I only recently noticed this because I wanted to manually move a file off the cache drive.

From a network connection (1GB) I can saturate the link in read/write to the cache, and similarly I get reasonable transfer speeds to the unraid array in read/write from the same network connection. 

 

Where do I even start trying to troubleshoot this problem?

You might start by posting up a diagnostics file with your next post.   'Tools'   >>>   'Diagnostics'

 

The file should contain a period when you had the issue and it would also be nice if you could specify the time window when the cache-to-array transfer was occurring.