Append additional backups to a removable disk

I"m using BE 2010 on a Windos 2011 SBS server (Windows 2008).  I'm using a USB disk to do periodic Archive backups.  The disk has the capacity to hold serveral of these archive backups.  The first backup to the disk goes fine but when I try to add an additional backup to the disk I get the error "e0000f16 - The operation failed to acquire the minimum number of drives and media needed.".

I have created a media set named Archive and have the overwrite protection period set to 1 hour and the append period set to 1 hour.  The Archive backup job uses that Archive media set.

What do I have to do to make this backup disk appendable?

Tx

Bill