FAT32 or NTFS...
Your 500Gb drive is probably formated using the FAT32 file system.
Your 2Tb just can't use that file system and must use the NTFS file system.
When using FAT12 (floppy), FAT16 (small hard drive) or FAT32 (medium to large hard drive), there are no area in use unless you are actively accessing a file.
When using the NTFS file system, there are ALWAYS two (2) areas that are constantly "in use": The Master File Table and the transactional journal.
Now, mainly because of those, the NTFS file system is very robust and forgiving. You can safely disconnect your 2Tb drive as long as there is no user file been accessed, it will be similar to a power failure. And, even if you do deliberatly disconnect as a file is writen, the only damage may affect the file been writen, you won't have any lost cluster, lost file or other errors, just an incomplete file that IS tagged as such.