Copying data to a USB external drive is much slower than expected.
I have a 6TB external RAID drive that holds my photos, and I'm trying to do an archive copy of this data to another external USB drive.
Both are HDD drives, formatted with HFS+. I have turned off the Spotlight indexing for both drives. I'm using a MacBookPro with fast USB-C ports, and USB 3.0 or better ports on both external drives. All cables are 3.0 (SS) cables or better.
The file transfer started at about 200MB/s then slowed to around 20MB/s and sometimes even less. I'm using the Activity Monitor - Disk to display the read/write speed.
I think the initial transfer speed was the write buffers on the target drive filling up. I'm expecting a real-life transfer speed for USB 3.0 to be around 100MB/s.
Any idea why this is running so slowly. How can I figure out the bottleneck?