Permission of external disk under Mac OS

Set your umask to 000 and reboot, then all files will save with full permissions for everyone. Here is the documentation http://support.apple.com/kb/ht2202

Generally speaking repairing permissions is a quick and easy task. head on over to /Applications/Utilities and look for Disk Utility. Launch it, select your HD and then click on repair permissions and let it do it's thing. That could take a couple of minutes or 15-20 depending on how messed up things are.

When I do this I usually quit out of ALL other apps and let disk utility do it's thing in peace. Then I run it again, and again until I get no more errors.

All of that assumes that it is a simple permissions error. It could be you are trying to modify a folder in a location that Apple does not want you changing things, the /Applications and /Applications/Utilities folder spring to mind.

If, however, these issues are on your desktop or in the folders Apple sets aside for your files (documents, music, pictures, etc.) then yes there is a problem and it MIGHT be more than permissions. Could be actual disk errors.

In Disk Utility you can "Verify Disk" but you can't "Repair Disk." That requires booting from install media, the recovery partition or the disk repair utility of your choice (my choice is Diskwarrior, YMMV).

If repairing permissions doesn't fix the issue AND you are working with folders Apple deigns to allow you to work with, use Disk Utility (or 3rd party tools) to repair the disk/file structure.