External HD

Did the Apple Store employee mention a few more details......things like.....

1) The AirPort Extreme must be the current "tower" shaped version of the product. (Older "flat or square" shaped AirPort Extreme models do not officially support Time Machine backups)

2) The external hard drive must be formatted in Mac OS Extended (Journaled) to work correctly with Time Machine. Unless the documentation that came with your hard drive specifically states that it is formatted for Mac, chances are the drive has been formatted to work with Windows.

Finally, your first complete backup of the Mac will go 3-5 times faster on average if you connect your Mac to one of the LAN <--> Ethernet ports on the back of the AirPort Extreme. Once you have the first backup done, you can switch over to wireless, since the subsequent "incremental" backups will be much smaller than the first complete backup.

I bought the G-Technology 2TB. Well, the HD isn't showing up on my MacBook.

Did the apple store sell you the USB drive?

I know they do sell some brands of USB drives.. G-Technology being one I think that did feature. If you bought it and confirm it is Mac formatted.. plug it into the Mac and check on disk utility.. reformat it if it is wrong.

However that is no guarantee it will work on the Airport Extreme which has a very limited range of devices that work.

Is the USB drive showing up under the disk tab in airport utility? It must otherwise it cannot be mounted in finder.

Sometimes firmware changes or USB chips change and a product that did work no longer does.. and visa versa.

The newest AE.. Gen6 has been more problematic than earlier ones with not detecting some USB drives.

I explained to the apple employee that the AirPort Extreme was the old flat model. He said it would work.

It might....but it won't be officially supported by Apple. Your chances will improve if you can mount the drive so that it appears on the Mac's desktop.......then see if Time Machine can choose the drive as a backup destination.

But....I would not recommend that you use this as your primary backup plan if you want to have a reliable backup system.

You can check the Apple support document linked below to confirm that only the current "tower" shaped version of the AirPort Extreme will officially support Time Machine backups at the USB drive of the AirPort.

Backup disks you can use with Time Machine - Apple Support

As Bob pointed out the flat model were not supported by Apple for TM backup.. for some people it worked fine.. but it is unreliable for others.. unfortunately unreliable backups makes backing up less than useful.

Time Machine to network target is not hugely reliable regardless of target. That doesn't mean Time Machine is the only show in town.. and I personally use Carbon Copy Cloner because TM is far from being trustworthy. CCC can also backup to a network target (albeit not a bootable clone). It is something worth a test. You can download and use CCC for a month trial.. and it is fairly priced with one license for all computers in the house.