Try first mounting the disk on the mac, can you write a file to it??
It shouldn't matter if the disk is is HFS+ or something else.
TImemachine should be able to mount the disk via SMB or AFP.
Try formatting the disk in Mac format first, see if that helps.
Got it working. I reformatted the HD as mac journal and now time machine can write to the drive. This time machine capability is indeed a nice addition to this router.
Thanks RMerlin and vdemarco