Just one other thought. Not sure from your description, but think your suggesting that drive A is at work while drive B is off site. Then on Friday you take in drive B, swap it with drive A and take A off site.
This method is really not good enough for a backup. For the whole of Friday you have both backup disks in the building. If there was a fire you would lose both!
Much better to have 3 disks. A, B and C. A in work, backing up things on a daily basis, waiting to be changed on the next Friday. On Friday bring in B swap it for A, take A off site. On the next Friday bring in C swap it for B. Etc etc. This is far more robust, at least one backup is off site all the time, and most of the time you have 2 backups off site which is good for redundancy. If everything goes tits up at work, worst case is you'll lose 1 weeks data.
Daily backups should also really be done. Everywhere I've worked the insurance was only valid if off site backups were carried out on a 24hr rolling basis for working days.
Oh, 1 last thought just to complicate things. In an ideal world backups should be archived at set intervals too. So every now and again you take a snapshot of the backups and archive it out of the system. This covers problems that may creep in and aren't noticed for a period of time. I think the classic is a virus infection that corrupts files, but is not spotted for a number of months. You can then refer to the archived copies, pre virus.