It appears you got a slightly different drive as you state its thinner. You gave no clue to what you got, model# and old model#. Offhand, I don't see where the problem lies, but be sure its "ordinated" in the proper mounting within the caddie. Normally I see that most laptop HDs will exchange w/o hassle, so this appears odd to me. Even though it's the same brand, compare the two HDs in hand and see what the physically differences are. The caddy is suppose to be part of the HD attached, then slide in as I would think it should go. Also, be sure that any minor part isn't still attached to old HD and has to swapped as well to the replacement HD besides the caddy.
I have an older Dell Inspiron 1545 that I'm fixing so it can be given to my pre-teen. The hard drive on it failed so I purchased one from ebay but found that even though it's the same brand, it doesn't fit in the caddy. It's a little bit thinner. I can slide the hard drive into the slot and then put the caddy in afterward but then I can't put the screws in to secure it. Does anyone know if there is some kind of a spacer or fix for this? I'm not opposed to purchasing a new caddy but not sure that will fix the problem.
Thanks for helping me out!