I've tried backing up my iTunes library to an external drive, but I can't drag the itunes file. How can I solve this?

I've never had a problem doing this, other than sometimes a file or two won't copy (error -36 I think, but it's been a while) and it will stop the whole process and make me do it piece by piece until I find the offending file.

What happens when you try to drag it over to the external drive? Error message, incomplete copy, doesn't do anything? How about copy/paste of the folder to the external drive?

Is it formatted as NTFS? Macs can't natively write to NTFS drives. They can read them fine, not write.

Drives must be formatted with a file system before they can be used. External drives typically come pre-formatted, often as FAT ("file allocation table", compatible with PCs and Macs) but could be NTFS. In a Mac store, you might find one formatted as HFS (also called "Mac OS Extended", not compatible with PCs) . If you were able to copy files to the drive from your PC without having to do something else first (i.e., reformat the drive), the drive came formatted as either NTFS or FAT.

Right click on the drive on your desktop and Get Info. "Format" is one of the things it will tell you. What does it say there? How about sharing and permissions at the bottom? What does it say there?

The short answer is no, you cannot change it. The long answer is you have two options:

1. Reformat the drive.. This will delete everything on the drive but you will be able to read and write afterward. If you choose FAT as the file system you can use it with your PC too. If it is only going to be used on Macs, format as Mac OS Extended Journaled.

2. There is a third party application that will let Macs write to NTFS drives. I do not know the name of it offhand but you should be able to find it with a search.