AFAIK, with the exception of WD and Toshiba 2.5" portables which have the USB interface integrated in the mainboard, there is no 2.5" or 3.5" drive in any external that is proprietary or locked in any way to prevent it from being used as an internal once formatted.
This is especially true for Mac users. You don't need to buy pre-formatted Mac externals. You can always format any external to whatever file system you choose.