After upgrading my PC from Windows XP to Windows 7, an external hard drive which was working fine on XP lost write permissions.

Found a solution:

1. Right click on the drive icon for the external drive.
2. Select Properties
3. Click Security Tab
4. Click Advanced Button
5. Click Owner Tab
6. Click Edit Button
7. Choose your account or administrators group, whichever you want to give permissions
8. Check 'Replace owner on subcontainers and objects'
9. Click OK

If this doesn’t resolve your problem, choose the user from “Group or user names” list, click “Edit”, and give appropriate permissions.

Hope this works for you!