Backup pre-Scan is slow, never ends, 26 hours later. . .

[Alan_Nolan]

I have a backup over a slow WAN link. It runs for 10 hours, but that's OK. 

If I run a backup pre-scan the pre-scan never ends, still running after 26 hours.

Any Ideas? Surely the remote agent should quickly run the scan, and just pass back the summary results? 

Thanks.