Western Digital External drive read-only under Gentoo - NTFS issue - solved
 

I bought a Western Digital 1TB external hard drive to use with a Gentoo build. It connected beautifully, mounted visibly but despite being mounted read/write any attempt to write to it produced the error "read-only file system". I chased a number of red herrings before I found that the drive comes with an NTFS filesystem and NTFS support in my kernel was set to read-only, which I think was a default setting. Simple fix was to install a different file system - as it was a new drive there was no old data to lose.

Glad to hear you figured it out.