Powershell – How to launch Explorer.exe with path \\server\c$ (different credential)

If you are using the -Credential argument, you may need to also specify the -FilePath argument:

$cred = Get-Credential
start-process -FilePath C:\WINDOWS\system32\cmd.exe -Credential $cred

As Joeqwerty said, it's usually accepted that you're going to be using NTLM auth with an external trust, and Kerberos with a forest trust.

However, it is possible to get Kerberos authentication working with an external domain, but there are conditions.

The trust has to be created using the fully qualified domain name (FQDN). Kerberos referral fails if the FQDN is missing from the Trusted Domain Object.

User name syntax is UPN and the UPN suffix is resolvable to a DC in DNS (implicit UPN)

UDP 389, UDP/TCP 88, and UDP/TCP 464 ports are open for the domain controllers in the user domain.

The server name in the trusting resource domain has to be the FQDN, and the domain suffix of the server name has to match the AD DS domain’s DNS FQDN.

If it's still causing you too much heartburn after that, I'd recommend switching to Negotiate authentication protected with SSL.

Some additional reading/references:

http://technet.microsoft.com/en-us/library/dd560679(WS.10).aspx

http://jorgequestforknowledge.wordpress.com/2011/09/07/kerberos-authentication-over-an-external-trust-is-it-possible-part-1/