I have physically and logically disconnected everything excepting the mouse and keyboard.
There is certainly no smartphone connected - and it's probably worth reiterating - there has been no change in my system set-up except for the movement from 4.3 to 4.4.
I don't see that there can be any cause other than what has actually changed, surely?
4.3 coped just fine with my set-up as it is, so the switch to 4.4 must be the root cause?
And if there has been anything I have missed disconnecting - it would have been in place when things were working perfectly under 4.3.
It is worth pointing out also that it's not an absolutely permanent hang. The import does happen - it's that it takes literally several hours, and that for the bulk of that time LR is frozen, unresponsive and shows as "Not Responding". Every 10 or 20 minutes, LR will appear to begin working again for a few seconds before plunging back into unresponsive.
I have tried monitoring CPU and Disk use during this using Task Manager - during the long unresponsive phases it does not appear to be that LR is using any CPU resource. During the few seconds it appears to respond, CPU use will not at any point go above 30% or so. So I don't think it's a resource issue.