Regarding people having success backing up to external drives (as have I) it seems to me that there is (as yet) unclear conditions that cause USB peripherals to go into some form of idle (or low power) state, which can cause USB drives to disconnect.
I have had diconnects with USB2 and USB3 drives and hubs alike, so can only assume that some sort of signal (or lack of even) is being picked up by Macbook which is significant enough to make it decide whether nor not a USB peripheral is mounted or not ... I almost think that Macbook can 'appear' to be carrying out a copy/backup process etc (screen not turning off etc) but underneath, the Macbook is deciding the USB peripheral is not being used (it is perhaps unaware that a USB drive disconnecting would be a bad thing, where as powering down a mouse wouldn't necessirly be a bad thing as it isn't copying data). This makes sense, if Apple assumed that mice and keyboards etc were to be plugged in (Firewire drives would not have this issue obviously - and don't appear to), but USB drives disconnecting could have been an oversight of handling idle (low-activity) USB peripheral use.
This post has quite alot of feedback that I honestly have not read in depth, so maybe my assumptions about non-drive USB peripherals is not 100%, but so far, it is looking consistent with Apple's own suggestion of not waking the Macbook with USB 'input peripheral' (notice their omission of waking a USB drive - maybe once system is idle, USB drives simply get powered down).
Have others tested what happens resource-wise, when a USB drive is disconnected!?
Can people having USB-drive diconnects, try using trackpad/keyboard on Macbook whilst copy/pasting etc to see if a disconnect happens!? Only one drive connected though (hubs have seperate power issues to deal with that might complicate findings). I am finding the drive has no troubles, but it would be nice if we could keep grouping together to find a repeatable solution.