SVN External failed due to authorization issues but can't figure out why.

Question

I've got a repo setup with 3 projects(1 per folder) and a 4th folder with some supporting DLL code in it. In each of the 3 projects I have setup svn:externals to this supporting DLL folder.
When I try to update any of the main projects I get the message (TortoiseSVN) "External Failed, unreadable path encountered, access denied."

Here is my repo layout:

_MY_REPO
--Project1/trunk (with svn:externals -r7 ^/SupportingDLLs/trunk Common)
--Project2/trunk (with svn:externals -r7 ^/SupportingDLLs/trunk Common)
--Project3/trunk (with svn:externals -r7 ^/SupportingDLLs/trunk Common)
--SupportingDLLs/trunk

With the following authz file I get the error message mentioned:
[/]
myUserName = rw

With this authz file it all works fine
[/]
* = r
myUserName = rw

What gives?

Answer 1

According to the Book, externals need a full URL (emphasis mine):

It can be set on any versioned directory, and its value is a multi-line table of subdirectories (relative to the versioned directory on which the property is set) and fully qualified, absolute Subversion repository URLs.

Answer 2

It's possible that you have

anon-access = none

in your conf/svnserve.conf file, and maybe you are accessing the external repository anonymously rather than with your username.