I ran into an issue when trying to paste data into a datasheet view:
All my columns seemed to be fine, and I know I had pasted items before. So I did a little research, and found it was a problem with multi-line text fields that were set for Enhanced Rich Text:
If you have a column in the datasheet view that is set to Enhanced Rich Text, you get the read-only error. If I change that to Rich Text (or Plain Text), then I can paste in the new information with no problem.
Jithendar Bathini 