iTunes library cannot be saved

I have an external drive for my iTunes library and keep getting the message "the iTunes library file cannot be saved. The required disk cannot be found."
I remember at the genius bar having to set this up but can't remember what I need to do.
Help.

Can you copy other files to and from this external disk?

Yes.
Originally we were going to move iPhoto to it as well but realized moving iTunes over freed up enough space on my hard drive so didn't have to.
I thought it might be to connection but I can access the library and have ejected and plugged back in and it works fine. I just keep getting this message pop up in a box and it won't go away.

So how are you trying to move the iTunes library? Are you following a specific set of instructions?

At what point exactly are you getting the error message?

I'm not trying to move it at all.
I went to the genius bar to get help with my slow mac. Their answer was to free up my hard drive by moving itunes and iphoto to an external drive.
Once we did that we realized just moving itunes freed up enough space and I could leave iphoto on my hard drive.
That was this past summer.
So now I'm noticing that once in a while I'll get this message that "the itunes library file cannot be saved. The required disk cannot be found".
I can open itunes, no problem. I can eject my external drive, no problem. I don't know what it's trying to save and why is it having a problem finding the disk? which I'm assuming that means the drive it's on.

Sorry must have misunderstood. Sounds like the disk is disconnecting or failing to cache read/writes.

It may just need a disk and permissions repair from disk utility. But, at worst, it may be a symptom of the enclosures hard disk controller failing.