Can read but can't write

I have just bought an Imac, 27" screen, etc. I plugged the external hard drive in, the one I used to use with my Windows PC, and it reads it no problem. I was able to transfer all my music and photos without a problem.

However, it won't let me copy anything to it. If I try to drag and drop from one window to another, it just tells me that I can't, with the grey no entry sign.

I am new to Macs, and so maybe I'm just not used to how it does things. Am I missing something, or is "can copy from but not copy to" a common thing to make me buy an Apple external drive?

its an NTFS formatted drive so you do not have write access to it

You can enable write access for OSX by installing MacFuse

macfuse - Project Hosting on Google Code

You new iMac with Snow Leopard can read and copy from a drive formatted to NTFS (your external hard drive), however it can not write to it. What I generally recommend for new users like yourself, is to purchase software that will allow your new iMac to read and write to NTFS. The best available software is Paragon NTFS current cost is $19.95.

I do not recommend using the free MacFuse as it's still buggy and can cause file corruption. Snow Leopard also has the ability to read and write from NTFS but by default is not turned on. Since you're new to OS X, it's best to purchase the Paragon software.