Cannot copy to external drive after reinstall

[Jake25uk]

I have had to re install my mac g4 and now I cannot copy files to my external USB hard drive, I get the error "500gb cannot be
Modified" it is formatted for mac and has always worked fine, any help would be much appreciated

 

[simsaladimbamba]

What are the permissions for that external HDD?

 

[Jake25uk]

Says read only, not sure how to change this