Without getting into remuxing with tsremux (I don't recommend tsmuxer, it produces incompliant output) or mkv (which i've never tried) -- I'm sure you'll find plenty of information on that -- what you could do is delete the m2ts files of the extra syou don't plan on watching again (rip directly to files, not to ISO). Make sure you don't delete any files that are required before the main movie starts, so you may want to rename the first and see if it still works. Don't worry about removing foreign language tracks or subtitles, nor files from any other folders -- the space you'd save that way is insignificant. As you become more proficient doing this, you may want to do more sophisticated stuff like removing forced previews and such.
Then just keep adding the largest available drives one by one when you run out of space until you reach you case's, mobo's, SATA card's or any other limitation. After that, remove those movies that you don't plan on watching in a long while -- that's pretty much how cache memories work in computers. By the time you fill your last drive, chances are that the largest available drive will be at least twice larger than the smallest you have. Replace that one. You may want to get an external enclose for it so you can reuse it. I've never had a hard drive I couldn't find a use for.
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