I think we need more information about how you are moving the library. Here's how I did it with my two MacBook Pros (one from 2011, the other from 2007):
First I went into my old iTunes and made sure I consolidated my iTunes library so that all of my music files were stored within one folder - the iTunes Music folder. You don't want to copy the folder leaving behind songs that are stored elsewhere. I lost a lot of music in the past that way.
I then connected the two laptops with a FireWire cable. I set the old laptop to Target Disk Mode (holding down the T as the laptop booted up). This turned the old laptop into basically a harddrive seen on my new laptop's desktop.
On my new laptop, I used Finder to go to my Home folder then to the Music folder and I deleted the entire iTunes folder (it was unused - so nothing was lost).
I then opened to the icon on my new laptop's desktop to work my way to the Music folder in my old Home directory and simply dragged the old iTunes folder to the Music folder on my laptop (using two Finder windows). Doing this with an external harddrive instead of another laptop would be essentially the same process.
This process is simply copying files, so I'm not sure where the issue of not having permission to copy files comes in.
Since you have a lot of files, I would also turn off power saving & security features that either put your hard disk to sleep or log you out of your laptop so that your transfer doesn't get interrupted. Just remember to turn them back on afterwards.
I suppose folks on this forum could enlighten us as to how files within one iTunes Music folder could have different permissions. I could see maybe having an issue with some of the files being read-only perhaps, but multiple users would have different music folders - they wouldn't share the same one. As for having songs purchased with a different iTunes account, that would only impact playing the songs or syncing them to an iPod or iPhone. You would have to authorize the laptop, via iTunes, for each account that you bought music with. But again, that shouldn't mess up the transfer. I authorized mine after I transferred the files.