Hi mate
I know this post is old but I saw it when looking something up and thought I would mention a couple of bits that might help. You said on your original post that the files had to go in the library if you are converting to optimised. Just in case you are not aware you can change that and put them wherever you like, external drive as would be normal. Just go to library settings before you import and change the location.
With the AVCHD files you have to put them on your hard drive or better save as archive first, you can then leave in place as you mentioned. This is a safe way of doing it and retaining file data as well.
To be honest the way I did things is based on the fact that I shoot both photo and film here and there so…. I right click a few times as stated above in previous post and copy the MTS files into my client folder template. I then import like any other file. I then immediately build proxy files. No matter how you get the AVCHD or MTS files in they are a pig to edit with. I assemble with the proxy and can normally do some good basic colour work with them before switching back to original for final work.
This is now a mute point anyway as I shoot everything with XACV anyway now and they are all MP4 files so no problem, FCPX rolls with fine. The quality is way better as well. I guess you know most of this now but just in case… Cheers