What is the best configuration for data transfer between two external hard drives?

If they are usb, use 2 usb ports directly connected to your computer and move other usb peripherals to a hub to make room. I have 2 WD Tb Mybooks and use the Smartware software for backup.

There is no real advantage. the data being transferred still uses the main computer as a transport device, even if you think the data is going directly from one external to the other external. The best thing, though, would be to disable the anti-virus software and firewall, disconnect any other usb devices, including internet, and not have any programs running. Also, make sure that the ports you are using are version 2 and not 1.1 as might be the case for some usb port cards or front usb ports, or hubs.

For such huge amounts of data I would consider temporarily installing the drives to ide or sata internal connections for the duration of the transfer.

There might be a problem or a slow response if your mac does not have a huge volume of “free space”. As I said previously, the data from A to C to B runs through the free space of C (ie mac). Just let it run overnight, i expect such a huge volume would take about 6 hours to transfer. Maybe boosting your ram might also help.

Thanks everyone. So the free space on your mac matters? I let it run overnight- I just want to understand for future transfers. Say I did have enough ports- as I do on another mac which isn’t here right now- is it faster to hook both of them up to the mac© than it is to daisychain? If I have a firewire 400 port and an 800 port is it better to hook up A or B to either.

All you can do is try the configuration. Trial & Error is the best way to do things, at least so that next time you will know, for sure, the best or better, way. By the way, how long did it take to transfer?

I don’t know because I did it overnight but it said it would take about 8 or 9 hours- this was three volumes of about 300–400 GB each.

Perhaps you should consider getting a PC rather than using a laptop so that you can, as @Dr_Lawrence suggested, attach them as slaves rather than externals. It would reduce the trnasfer rate by about a quarter. Not that much time saved but if you need your laptop and it’s all rigged up, you cannot stop or cancel the copy transfer without losing or possibly corrupting files.