While Windows systems suffer from the MBR's constraints, Macs and 64-bit Linux machines have no issues using or booting from 3TB drives. Apple has supported EFI/GPT since it switched to Intel CPUs. Most 32-bit Linux distributions also support 3TB drives, without requiring an EFI/UEFI BIOS.

You can use the full capacity of a 3TB drive via USB--with a drive enclosure that supports it. The SATA-to-USB bridge chip inside the enclosure takes care of any addressing issues. This is why the as an external model--a switch from the usual pattern of internal units shipping first.

Although you shouldn't have any issues attaching a prepackaged 3TB external drive, you may have problems if you buy a 3TB internal drive and try to add it to a USB-connected enclosure yourself. Read the fine print: Many of the bare enclosures say they support 3TB, but not all of them do.

If you have 64-bit Vista, 64-bit Windows 7, a Mac, 64-bit Linux, or in some cases 32-bit Linux, check to see if your system's BIOS supports 3TB. If doesn't, you'll need to obtain a BIOS update or to buy a newer motherboard that supports 3TB.