Excellent point. It could be a 6.0 Gbit/s model in the AP TC, but I doubt it. Your point about interface overhead is a good one.
An external disk plugged into the AP has several advantages:
- You can plug it in directly to the computer for the first backup or loading files on to it over a higher speed interface, then connect it to the AP and perform your incremental backups or changes over wifi and the slower interface.
- You can replace or upgrade it without having to void the AP's warranty.
- You can fill it with a back up, unplug it, and move it off-site.
- You can bring it with you without having to disconnect your network.
I'd definitely lean toward the cheaper AP Extreme and your own external USB disk(s).
I am pretty sure you can't plug the external HDD to Mac for the first back up then use wifi to do the incremental backup. I first thought this is possible and did this but when I plug the HDD back to Airport Extreme, it would do a complete backup again.
I looked into the HDD and see that the backup files are totally different. If the back up is done through wifi then there will only be one file (something.sparseimage) which contains the entire back up. You cannot pull single files out by locating it in different locations. You will need to enter TimeMachines to pull files that way. Here is how it looks like (http://pondini.org/TM/17_files/E2b Sparse Bundles via Finder.jpg)
If you were to back up with cables (USB, Firewire, Thunberbold) then you will be able to pull the files by just locating the files in the HDD. The files you see is call backups.backupdb which can you go in further to see different time stamp and whatever files that are backup. Here is how it looks like (https://discussions.apple.com/servl...-600/Screen+Shot+2013-01-03+at+8.18.36+PM.png).
You may want to read this (https://discussions.apple.com/thread/3734638).
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