>Create a share on a server with the share name containing a space, such as the legal share name "A Space". Assuming that your server is named MyServer, issue the following commands:
>
>CD "\\MyServer\A Space"
>? SYS(5)
>
>Notice that there is now a space in SYS(5), and since you've referenced it by UNC, there's no colon, and it's more than two characters in length.
>
>Try it, you won't be a happy critter at all at all. If you designate the starting folder in the shortcut as a UNC, you'll get bit - the OS sees the UNC and relies on the UNC, not a transient drive mapping...

That's clarified part of the issue of sys(5), & the possibility of spaces in the value returned, again thanks.

The second part of the problem, as stated, was that the "set default to sys(5)" would not compile - now with the above would you expect a compile time or run-time error.