OK iMovie needs the entire folder structure of your camera to import AVCHD, so in the future, you can import directly from you Sony. Also, in the future, on the iMovie import screen, you will see an option to "Create Archive". This will create an exact copy of the contents of your camera, including the folder structure, so you will have a good backup of exactly what you shot.
So now you have the problem of how to use all the clips you stored on your external hard drive. These are probably .mts or .m2ts clips. There are a couple of third party tools that can do this for you. I will mention the name brands that many people have mentioned. There are also some free tools out there, but I don''t know as much about them.
1. Voltaic HD will take your raw files and convert them to Apple Intermediate Codec. These can then be used in iMovie.
2. ClipWrap is another tool that can take your raw files and convert them to AVCHD. With clipwrap, you also have the option of "re-wrapping" the AVCHD files into an h.264 container that iMovie can understand. This option is attractive because it is almost instantaneous. However, AVCHD and h.264 are both highly compressed codecs, and unless you have a very powerful Mac, you will get better editing performance by converting to Apple Intermediate Codec first. (This gets to your second question.)
You can think of it like this... The highly compressed frames of video must be converted into full frames at some point. By converting to AIC, you convert in advance. By leaving them as h.264, the conversion must be don on the fly as you are rendering your finished movie. Rendering puts a heavy load on your system resources, and adding the decompression adds a significant amount to the load.
Once you have converted the files, you can import to iMovie by using the FILE/IMPORT MOVIES command in the iMovie menu.
OK. You should be aware that you can reformat your current drives to the apple format. It still might be a good idea to buy a new drive, because reformatting would cause you to lose any data that is currently on the drive.
But if you have an empty drive, you can reformat it using Disk Utility (in your Applications/Utilities folder).
In Disk Utility, select the drive. Then choose "Partition". Give it a single partition and a name (unless you need two for some reason). For Format, choose Mac OS Extended (journaled). For Partition Map Scheme, choose GUID Partition Table (unless you are on a very old Power PC Mac). Then Apply.