Q: disk permission

disk utility does not modify permissions.

what is the disk formatted as?

select the HD icon and press CMD+i

if the drive is NTFS you may not write to it in OS X (by default) and if you could in the past it was due to 3rd party software which is possibly incompatible so it would become READ ONLY with any account.

If you need to be able to write to this disk you will need to purchase 3rd party software if it is not available from the vendor for this purpose or backup the disk and reformat to a mac compatible format. While there is a terminal command to allow NTFS drives to be written by default on mac the process has been cited as causing data loss to the drive and is not advised.

if the drive is Mac OS Extended (Journaled)

select the drive icon with CMD+i

you have two possible choices:

the first will modify your permissions and could potentially cause a problem if it is implemented incorrectly:

go to the Sharing and Permissions tab and extend the details tab (triangle shape)

check either your account name under the tab so it allows Read & Write

click on the gear icon and go to Apple to enclosed items...

(also note an NTFS volume will not have any Mac permissions available as they are not stored on the drive)

or if the drive is external you can uncheck

Ignore ownership on this volume

and you should need do nothing more

it is highly advised regardless of what action you take to back up this volume now. If the volume is formatted NTFS you will be unable to do this with Time Machine.

You always set permission via "get info" - go to bottom, unlock, proceed.

You might a,so perform disk first aid, which (now, in Apple's effort ti dumb down things beyond where they should be dumbed) includes correcting permissions.

Grant

Permissions repair is no longer necessary in OS 10.11 as a result they are no longer included with disk utility for os 10.11

First Aid from Disk Utility will check the validity of volume, catalog and the extended attributes but does not do anything with permissions.

Repair disk permissions with Disk Utility - Apple Support

I don' buy that/ In fact, its not possible.

They may "protect" them, but errors still occur.

So Apple is not telling us everything. They MUST have algorithms that check permissions based on the file, its location, and other environmental factors. If not, the tsunami of issues will begin soon.

Grant

Under the System Integration Policy in OS X 10.11 the permissions are only set during trusted software installs and updates by developers who (hopefully) understand the process that enables this but access to permissions is no longer given to the owner. It is no longer available in Disk Utility and not administered in a Disk Repair for this reason.

Select the EHD icon on the Desktop and type Command + i to bring up the Info window.  In the Sharing and Permissions section check the Ignore ownership on this volume checkbox.  To get to the checkbox you'll have to click on the locked padlock and enter your Admin password.

Just FYI - disk utility will reformat any disk - windows, Mac or UNIX