I second phreakout and Bunsen's suggestion to ensure the OS 9 drivers checkbox is ticked. Because Apple would rather pretend that Mac OS 9 never existed, when you format a volume in Mac OS X (any version), it does not actually install the drivers (software that allows you to access the volume) for Mac OS 9 systems by default - you must ensure that checkbox is ticked if you plan to use the drive on an OS 9 (or below) system.
Drive Setup for Mac OS 9 (and below) run in the older versions of the Mac OS and therefore automatically install the required drivers.....really, if you're planning on using the drive with both an OS 9 Mac and an OS X Mac on a regular basis, I'd format the drive on the OS 9 Mac - so long as you format the drive in HFS+ (Mac OS Extended Format) rather than HFS (Mac OS Standard Format), it'll have everything it needs built in for you to use it on either a Mac OS 9 or Mac OS X system - unlike Mac OS 9, OS X does not require any specific drivers or whatnot.
Drive Setup for Mac OS 9 (and below) run in the older versions of the Mac OS and therefore automatically install the required drivers.....really, if you're planning on using the drive with both an OS 9 Mac and an OS X Mac on a regular basis, I'd format the drive on the OS 9 Mac - so long as you format the drive in HFS+ (Mac OS Extended Format) rather than HFS (Mac OS Standard Format), it'll have everything it needs built in for you to use it on either a Mac OS 9 or Mac OS X system - unlike Mac OS 9, OS X does not require any specific drivers or whatnot.