This may be of general interest to some here.
I have recently bought a few terrabyte drives for storage. A few weeks ago I had occasion to transfer files from an existing 2Tb drive to a new 3Tb drive - the old one got full up!
Since I was effecting the transfer on a Mint machine that would not thereafter be using the drives, I plugged both drives into the front-panel USB ports. Everything went well and within about a day all the files had been transferred.
Yesterday I had occasion again to transfer files from the 2Tb drive to another new 3Tb drive - similar files but a different mix.
Since the new drive was to be a permanent external of the computer doing the transfer, and the 2Tb was now a fixture on that machine, they were both plugged into the same external powered USB hub.
Expecting to run the transfer over-night, I set the transfer going and only checked sporadically on progress. About every half-hour (sometimes less) I found the transfer had stopped and there were four new caja windows open (there are four external drives in total and new caja windows open up when an external drive starts up: hence the 4 drives had "disconnected" and "re-connected"). I left the system running over-night without much hope and when I looked again today transfer had again aborted.
Remembering the previous successful transfer setup, I removed the target 3Tb drive from the hub and plugged it into a front-panel USB socket. So far all is well. Why this works, even with the 2Tb drive on the powered hub, I have only a partial idea: that the powered hub introduces a delay of some kind that is not affecting the 2Tb drive as such. (I have no doubt the rear USB sockets would have sufficed had I any to spare.)
Other notes: I disconnected and effectively removed two of the four drives (1Tb and 3Tb) but that made no difference. Only the 1Tb drive draws all its power from the USB socket: the 2TB and 3TB drives have external mains-powered adapters plugged into spike-suppression sockets.
I recall a similar situation with a 400Gb and two 200Gb external drives on a Ubuntu 10.04 machine - again on a powered hub (all are 7-port USB2 Triangle powered hubs - I have 5 in all). These drives were fine until I added a fourth drive, which sent them mental; removing the new drive settled them down again.
So: if you want to maintain long-term connection during transfer to external drives, connect them to the computer, not to a powered hub (I have no idea what a non-powered hub would do). This does not seem to affect ordinary backups, which are of relatively short duration.
Further reference: all transfers made using LuckyBackup. I find it easy to use yet reasonably versatile for all my backup and major copying needs.
Another note: powered hubs are ok in general for permanently-powered machines. If you expect, say, a laptop to power-down external drives when closed you have to use a non-powered hub or connect direct to the laptop. Think about it: the drive's controlling power comes from the USB socket, which for a powered hub is always up.