USB hub
As a result of reorganizing my office during the installation of my new computer, my backup system (which is is running inside a fireproof safe) is no longer close to my desktop computer, so I installed a USB hub between them. That was a very bad idea as a the full backup took 12 hours instead of the 9 hours that it took with my previous computer and a direct connection.
So I replaced the USB hub by a one meter extension cable (two cables in fact as the backup disk is connected with a USB Y cable, and extending the Y cable this way lowers the potential voltage drop). Now the backup time is 6.5 hours, nearly half of what it was with the USB hub (the drop in backup time from 9 hours to 6.5 hours being the result of the faster computer).
I knew that USB hubs can create problem (I have at least one device, an HD DVR, that does not work if connected through an USB hub), but slowing things down that much was a surprise.
As I was doing this I tried to verify if the Y cable was really needed. I used a USB monitor that confirmed that from time to time the disk will draw 0.7A, slightly more than the 0.5A limit.
My Passport disk
As I was testing various configurations for my backup drive, I confirmed that this drive was never spinning down. I suspected this and because the CHECK POWER MODE is not supported on this disk I have to unmount and disconnect the disk each time I want to open my safe (The disk being stored in the door of the safe). I tried to fix this without success so far, but I was able to at least fix one problem in the smartctl configuration by having this modification in the /var/lib/smartmontools/drivedb.h file:
{ "USB: WD My Passport USB 3.0; ",
"0x1058:0x07[4a]8",
"",
"",
- ""
+ "-d sat,16"
},