ubd was eating up all network bandwidth after latest upgrade.

The recovery partition does not have mountain lion on it, only an app to download a fresh copy (that si why it is some 600MB instead of 3GB.) You can also download the installer from the app store and make a USB key from it by looking inside the item it downloaded, finding a MDG file called Installer ESD, and using disk utility to burn it to DVD or use the restore to a USB stick.

Ah! Thanks. That makes much more sense now. Well, maybe tomorrow night is just gonna be a bust while I wait for a few 100 GB to copy around.

Actually I think you're right. I must take back my previous conclusion: the issue seems to have resolved itself for me now. And I haven't restarted the computer – only turned off and on the Documents & Data checkbox in iCloud in System Preferences as I described. You may well have been right, that it was a corrupted file which has been replaced by doing this, and the massive activity I was seeing (and inaccurately describing as going on and on) was just the initial redownloading of iCoud documents and data.

All seems well now, no fresh install, not even a restart. Phew! Let's hope it lasts.

You could try spending less time in restaurants 😝

Wow. Thanks for the advice, stillripple. I finally risked turning iCloud documents & data off and back on, but I double checked that I had backs ups of all my data on another Mac. All the data came back; at first it was a rush, MB/s of data coming in from the cloud. It died down to the same 300 kB/s hemorage I had seen before. But three days later, yep, the constant 300 kB/s drain is gone. I'm now seeing peaks of 59 kB/s every 30 seconds or so with a constant background of less than 1kB/s. I'm crossing my fingers the constant drain is gone for good.

Nope. The data drain is back, has been for a few weaks. Do I really have to turn iCloud off and back on each month to stop the perpetual drain?