Have you looked at Crashplan? It's been discussed a number of places on TR and the forums before so maybe you've considered and ruled it out but I thought I would make its case for you.
1. You can set it to backup changes as frequently as every minute - i think that would meet your requirements
2. Network drive as source or target? I think you can overcome both (possibly) but I'm not sure how the unofficial solutions meet your needs:http://support.crashplan.com/doku.php/r ... ped_driveshttp://virtualj.net/20120126/use-crashp ... -drive-nashttp://homeservershow.com/forums/index. ... re-solved/
3. Maybe I'm misunderstanding what you want, but this one might be a hard target to hit. So you want to track all changes (with versioning on all?) and only backup when you connect a drive? I don't know if it works how you want - you can certainly have multiple folder destinations on different drives and when you connect them, the program will sync any changes between the present and what is stored on the destination and make block level changes. If you want to track any/all changes I think you're going to need some kind of always on/connect backup destination to sync changes with. For example, I use it at work and it backs up our main database, quickbooks files, etc. to a local drive as well as the cloud every 15 minutes and there is a larger set of items that get backed up overnight. The backup takes all of 3 minutes to make the block level changes to what are 800mb+ files. If you pay for Crashplan+ you get unlimited cloud backup (which you appear to not need) but also the ability to have multiple backup sets. I wonder if you need all 2TB of data backed up/monitored constantly or if you have a smaller data set that could use an always on destination with another set that could be on a daily schedule or as you connect the other drives (more like an archive/cold storage set). The external drives I have (WD and Seagate) are both smart about spinning down when they haven't been accessed for a while if your concern is power usage.
4. I've run it under XP, Vista, 7, 8, Server 2003, and 2012. I had an issue with my 2012 install and support has been great and I think it's fixed now (something with Java and a reinstall seems to have fixed it).
5. Tons of functionality in the free edition (including offsite backup to other computers, friends, family, etc.) and even more with unlimited cloud storage in the paid version.
6. Completely optional