Some laptops (usually the more expensive ones) come with a free slot, which may bear a second hard disk or CD/DVD drive. Every manufacturer seems to name it differently, names like MultiBay(TM) and SelectBay(TM) are common. Different Linux tools are available to handle these hot-swapping devices.
thotswap is part of the Toshiba(tm) Linux Utilities it makes it possible to hotswap devices in the SelectBay.
Hotswap is a utility to register and deregister hotswappable IDE hardware. It is written to be used on Laptops with some sort of hardware bay to remove the module from the machine without rebooting it. Note that this utility is not required to insert or remove batteries or floppy disk drives; only for IDE devices.
The hard disk management tool hdparm also comes with a hot swap option.
Some bays can (in some cases only) carry a second battery. Currently I don't know how Linux can handle this. For example are there any tools, which show battery stats for the second battery?