I'm not sure it really matters which drive goes where since if one of them fails the unit does not work. You will need to "marry" the 160GB drive to the 30GB drive. I would recommend hooking up both drives per Hinsdale, making a backup of the 30GB then restoring the image to the new drive, then mfsadd them to work together. You may have to mess with unlocking the original Sony drive per Hinsdale. They can get locked again after unlocking so it is always wise to check.
TiVo 240060 with 200GB Seagate (wireless)
Humax DRT800 with 160GB Seagate (wired)
Anywhere from 0-10 misc. standalones waiting for upgrades for resale on eBay.