I don't know much about how Windows implements its filesystems, but under many versions of Unix, it's possible for the drive to appear to be totally inactive, yet not have data flushed to it. I wouldn't be at all surprised if Windows did it the same way. That is, there's no way to know if the writing is done except to tell it explicitly to finish. This is known as "lazy writing", and I've heard a lot of complaints about Microsoft's implementation of it, although I haven't investigated it enough to know what the complaints are specifically.

Regardless, just get one of the combo enclosures and buy two cables. That way you'll be able to see if one is better than the other (I doubt it), and have greater interoperability to boot.