You can always use another utility to convert files at a later date (Roxio Toast and VoltaicHD are two tools you can use) but even though I have VoltaicHD, I see it more an an emergency tool; I'd much rather use Final Cut to do the import. It's faster too.
One of the best ways to archive AVCHD files - and keep them still accessible by Final Cut - is to use the Disk Utility to create a disk image file (.dmg) This is easy to do, and preserves everything so that Final Cut will still be able to read the files.
Making a Disk Image
To make a disk image. Connect your camera to the computer using a USB, or if the camera uses removeable media, you can use a memory card reader to mount the card. The important thing is to have the media appear as a drive in the Finder.
Go to the Applications folder, open Utilities, and launch Disk Utility.
In the left hand column, select the card (or camera), and then choose Edit > New > Disk Image from (the name of the Device.) Note that if it says “Select a Device” then you haven’t selected the device you want to create a disk image of.
A Save As dialog box appears, asking you to name the file and choose a location. You can also choose the Image Format, with Compressed being the default.
Compressed or Read-only
The video files are already heavily compressed, so using the Compression option is simply adding compression ontop of that (the compression savings probably only have any effect with the support files.) I found that when compressing a 3GB AVCHD device, there was only a 40MB savings (just over 1%.) I suspect that number will be pretty consistent no matter how big the actual archive.
On the other hand, it appears that a compressed disk image slows down access time; the OS decompresses the files as you access them from the Disk Image, not when it "mounts them."
I did a simple test with a compressed and a read-only archive, and with a 19 second clip I found a 10% difference in the time it took for Final Cut to transcode the file. (20.6 vs 22.9 seconds.)
So I would recommend using Read-only.
Click Save, and the disk image will be created.
Mounting the Disk Image
Once you’ve created the Disk Image, you just double click the .dmg file to open it. A dialog will open saying that it is verifying the disk image. Unless you have reason to believe the file was corrupted (i.e. copied off a bad DVD backup) I don’t think you need to perform the verification. Click Skip.
A warning appears that the file could be damaged, but that’s just there because .dmg files are often used for sending software via the internet. Click Open.
The “disk” will mount, and you can launch Final Cut and import away, just like you did before. Dismount the “device” just like a regular removable drive.
And don’t forget to make a backup of the .dmg file!