Yosemite Server Backup 8.8 and newer
The full text of error 4474 is:
"Write access has been denied. Please check your permissions to the specified folder."
This error posts when configuring a Virtual Library Device in Yosemite Server Backup. The error means that Yosemite is unable to correctly read from and write to the storage folder location specified. Often this is due to the location of this storage folder being a UNC path or mapped drive. The security credentials Yosemite Server Backup is using to access this location are based on the credentials the Primary instance of YSB is using.
On a Windows system this defaults to the local "System" account and is specified on the properties of the Yosemite Server Backup service. If the Yosemite Server Backup service is stopped and the Yosemite GUI is opened then the GUI will become the Primary instance and will inherit the credentials of the user currently logged on at the time.
Providing Yosemite Server Backup with credentials that have the ability to both read and write at the location specified on the storage folder will be required to resolve this error. One option is to modify the user that the Yosemite Service starts with. This can be done by going to Start > Run > services.msc. Then locating the Yosemite Server Backup service from the list and selecting Properties from the right click context menu. On the properties window there should be a Log on tab. It is on the Log on tab that these credentials can be modified.
Link to this page: