Restricting access to an external hard drive

If the external drive has its owner set to an administrator account, and Others access set to read-only, then normal file protection will apply to a non-administrator guest account. Of course there is no protection against the guest connecting their own laptop to the external drive. For that, you need to keep the data encrypted. You could use hardware encrypting, and dismount the drive after each backup. A key is required to mount it.
<http://www.radtech.us/Products/Impact35Encrypted.aspx>

For some reason, admin authentication is only needed to uncheck the "ignore ownership on this volume" checkmark, but anyone can check it, which pretty much removes all the regular file permissions on an external volume.

For some reason, admin authentication is only needed to uncheck the "ignore ownership on this volume" checkmark, but anyone can check it, which pretty much removes all the regular file permissions on an external volume.

At least on my system, if I check that box from a non-administrator account, it asks to authenticate. If I don't give it an administrator account and password, the box appears to be set, but if you close the Info window and do Get-Info again, you will see it is not really set.

Hmmm - no matter what I've tried, any user (admin or not) can check that box without any authentication and get access to files on the external drive. Even though the Get Info looks OK and folders have the no entry decal, they can still be accessed. About the only thing that seems to work is if the permissions on the external drive itself are set to no access by others, then the drive does not get mounted at all in an account without user or group access permissions.

Disk Utility doesn't have the option of using the super user to mount a disk, but the Terminal does via the diskutil utility (DU uses diskutil). Give this a try:

1) Open the Terminal.app from an admin account
2) Type the command: diskutil list (this will list all of your disks)
3) Look for the disk you need to mount, for example, /dev/disk1 (there will be various partitions, but this is the main disk number)
4) Type the command: sudo diskutil mountDisk /dev/disk1 (using the disk number you found, of course)
5) You will be prompted for your admin password, then all partitions on the disk should mount
6) Change the permissions

None of the help here worked for me.
Applecare solved mine by deleting/trashing com.apple.finder.plist & com.apple.desktop.plist from User preferences then restart.
user/library/preferences.

Worked for me where nothing else here did.

You can't really restrict access to the external drive, but you can use an encrypted disk image to password protect your files.

Log in as whoever owns the external drive (probably your admin account), and it should mount - then you can change the permissions bas desired.