I'm trying to replace a failed Hitachi 80 G with a Seagate 320 G drive but neither the new or the old drives will reinsert. I removed the old drive and reinserted it as a test with no difficulty. I then switched the coverplates and attempted to insert the new drive. It meets resistance. THere appears to be some piece of black plastic or rubber in the way. I attempted to reinsert the old drive. No luck there. Any advice is most welcome.