Time Machine Backup not recognizing old backups?

Don't think attaching an external drive to the USB port of the Extreme is supported for Time Machine

 

So if you attach an external hard drive to an Airpirt Extreme you are able to do a wirelessTime machine backup? Just curious because I have an Extreme on the way..

 

That's correct. When connected via USB, TM creates directories on the disk. But when served over the network, TM maps a "sparsebundle" from the disk (a type of disk image) and writes te data within there. In other words, different formats.

There are probably instructions on how to copy the old data on the Internet. But you'll need to copy the old data to the sparsebundle if you want to see it.