Q: why does finder ask for password while logged in as admin

Are these files and documents in your home folder tree?  Or are these files somewhere else on the system?

The "admin" user vs a "standard" user.

The admin user is a member of a few more groups:

  33(_appstore)

  79(_appserverusr)

  80(admin)

  81(_appserveradm)

  98(_lpadmin)

  204(_developer) # this could be because I installed XCode

The more interesting one, being the 'admin' group, which gives me the right to run the 'sudo' command and to enter my password to authorization dialog boxes.

If and ONLY if a file has one of the above groups listed as the file's group, will that file give an admin user permissions group permission access that a standard user may not be able to get.

Otherwise an admin user and a standard user are not treated any differently during normal usage.

So if the files you are trying to access are suppose to be your own (as in they are in your home folder tree), then that is a reason to wonder why your account cannot do anything to them without entering your admin password.

Select the file, and then Finder -> File -> Get Info -> Sharing & Permissions.  Who is the file's owner and what permissions have been granted to the differnet 'name' entries?

If you want to use the Terminal, you can use the command

/bin/ls -dleO@ /path/to/the/file

If they are files outside your home folder tree, then they most likely are owned by some other OS X system account and you need elevated privileges to do things with those files, hence the need to enter your admin password to an authorization dialog box.

So it is important to know what files you are being asked to authenticate.

Check if the external disk is set to Owners Enabled?  You can use Disk Utility to find this information

If "Yes", then you need to see if the other external disk is "Yes" or "No".

If it is "No", then I am surprised you are being asked to authenticate.

If it is "Yes", then it matters who the owner of the files are and the permissions on those files.  Also if the other external is "No" and your problem external is "Yes" then chances are changing yours to "No" will cure the problem.

At this point you need to know the setting of the "Owners Enabled", and if "Yes" for both externals, what the ownership and permissions of the files that work vs the ones that do not work.

Changing the Owners Enabled should be an option in Disk Utility, but it is also available from a Terminal session diskutil command line.

User File Ownership - Reset   see post by Linc Davis

You may need to rebuild permissions on your user account. To do this,boot to your Recovery partition (holding down the Command and R keys while booting) and open Terminal from the Utilities menu. In Terminal, type:  ‘resetpassword’ (without the ’s), hit return, and select the admin user. You are not going to reset your password. Click on the icon for your Macs hard drive at the top. From the drop down below it select the user account which is having issues. At the bottom of the window, you'll see an area labeled Restore Home Directory Permissions and ACLs. Click the reset button there. The process takes a few minutes. When complete, restart.   

Repair User Permissions

With Owner Enabled external disks, file ownership and permissions are followed, which means even an admin account needs to be granted permission to access a file, or in the case of an Admin if not granted normal access, it must get elevated privileges by entering its password.

If you start having this problem again, you should look at the owner and permissions of the file in question and see who is the owner and what access is allowed for you.

Did you authenticate for the first transfer, or cancel?

If you did the authentication, then that authentication will remain in effect for 5 minutes.

Does that explain what you are seeing with respect to list view vs icon view?

If you want to get to the bottom of why you are being asked for authentication you will need to look at the ownership and permissions on the destination folder, as well as the source folder and source files to be sure you are are not allowed to move or copy things or if you need authentication to get elevated privileges.

Hopefully you did NOT Apply permissions to enclosed folder on your System disk.  This is a good way to break your system.

If you want to play with permissions on the external disks (assuming they are NOT bootable clones), that is fine

If you want to play with permissions in your own home folder, that is fine (but there are a few folders that have specific permissions, but if you mess with them it is unlikely to really affect you, as very few people actually use their Macs in a multi-user environment).

Restoring default permissions.  Disk Utility -> Repair Permissions should do it.