Jitters and a spinning beachball!

I using FCP X to edit product promos, I followed some advice to store the events and finished projects on an external drive. I have a LaCie 2big 6Tb drive so I could easily accommodate the raw footage. Launching FCP X, and then accessing the data to the drive, I get the spinning ball and its very jittery and freezing a lot.
I have 16 Gb of RAM so there plenty of processing speed, can anyone shed some light on this for me, as its totally unworkable right now?

Need complete specs of media and system.

Hi Tom,
iMac is a 27" with 16gb of RAM, I use a LaCie drive to store all my events and projects on. I bought the drive because I filled my iMac up to the brim of 1Tb of data which why I thought the iMac had become sluggish.
But I have experienced the same jitteriness and beachball on my MacBook Pro which has 8gb RAM. Both computers are unworkable with FCP X right now! I didn't have an issue at all one month ago. Not sure why I am now having problems.

"Need complete specs of media"

Can you answer this please.

What version of the software and OS are you using?

Is that LaCie drive Firewire or Thunderbolt?
Did you clear that internal drive off so it has at least 15% of its total capacity as free space?

RAM does not increase processing speed or power, only the ability to load full databases and temp clips into RAM. Don't get hung up on RAM, not what your issue is.

And then what Tom asked, very important.

Both computers are using the latest Mountain lion. The drives are thunderbolt.

Sorry that my previous reply was a bit short, but I needed to race to South Wales. The OS is up to date with Mountain Lion, I did the update the other day. I will say though that today it has been ok on the MBP. I am however editing footage shot from my own camera and the editing experience has been a joy. The footage in question was from multiple cameras, shot by a multiple of cameras, of which I think one was tape. Thats the only reasonable answer that I can give.
I suspect that my issue isn't solved though.

Are you optimizing upon Import?
I would STRONGLY recommend optimizing if you're doing muliclip edits.

how do I optimise the import

When you import your video, there is a checkbox for Optimize Media.

If it is already imported, you can select one or more clips in the Browser, right-click, select Transcode Media from the pop-up menu.

Thanks Ben, I will give that a bash this afternoon.