Problem with External HDD

I'm using a Western Digital 500 GB Passport HDD with time Machine on my MacBook Pro. As I've had the flu and been working from the sofa, I had the great idea of connecting it to the USB hub on my Airport Extreme and using it wirelessly. I successfully saved and deleted files and apps to and from the external HDD. The problem is I went to run Time Machine and now it thinks it's a different HDD or something and says oldest backup "none" and latest backup "none". Why doesn't it see the backup that's already present? This is probably a mistake I would have encountered on any system, considering I don't understand much about wireless and networks. I don't want to have two full backups on the external! Is there any way I can make it work wired and wirelessly, or at least undo this and put things back to normal? Thanks!!

First I am surprised that a USB powered hard drive worked at all on the Extreme for TM.
Second the only way I know you can do it and be wired and wireless is by ethernet connection to AEBS when not done wirelessly as there is a difference in how it communicates with TM.

Oh yeah, forgot to mention there is also now a folder that appeared on the external HDD called "MacBook Pro.sparsebundle". At this point all I want is to undo whatever I've done by using this HDD wirelessly. I just want to go back to strictly using it wired as my backup disk and everything normal, where it recognizes all my existing backup data. Can anyone advise me if this is possible? Thanks!