Redundancy - Back up to two Time Capsules, or TC to Spare Drive
1) Backing up an iMac to a Time Capsule (networked) and also backing up the same iMac to an external drive on the iMac via (USB or Firewire) alternating between the two destinations.
or
2) Backing up the iMac to the Time Capsule, then archiving the Time Capsule to an external USB disk connected to the Time Capsule using the Airport Utility.
1) One question is if I use Time Machine to back up to both the Time Capsule and external disk alternately, when it calculates the differential, does it have to do so from scratch each time for each disk as they are chosen, or does it keep some type of snapshot for each disk, and just choose from there? (I'd rather not put a huge load on the respective disk, and the internal hard drive every time a Time Machine disk is chosen.)
2) If one archives the Time Capsule to an external disk via the Time Capsule USB port, is this less reliable than the method above (ie: if the Sparsebundle on the Time Capsule becomes corrupted, does the Airport Utility transfer merely copy the corrupted data?) Also, isn't this archive rather intensive on the TC and external drive disks, copying its entire contents each time, or is it differential?
3) Pros and Cons of each method?
My iMac has Leopard, but I can update to Snow Leopard before putting the redundant backups in place.
iMac 24 and Macbook Aluminum, Mac OS X (10.5.8)