Access to the specified file is denied

I recently had to switch machines that I was saving backups to. Formerly, I saved Windows backup images to a hard drive shared via Samba in Ubuntu. That worked fine, until the physical drive died.
Now I have a Windows 7 machine with an extra internal HDD, and I cannot get Acronis to backup up my main PC.
Both machines are Windows 7. They're in the same homegroup. I partitioned and formatted the new HDD, and checked that full permissions are active. I can create folders on the drive from my main PC, so those permissions seem to be working. Yet Acronis refuses to back up to it. Here's the error screenshot.
Could this be a firewall issue or something?