I have two Windows Server. First (a) is a W2012 Essentials. Second (b) is a W2012 R2 Standard running on top of XenServer 7.0. I have attached an external USB disk to server a and created a virtual iSCSI disk on it that is shared to initiator on server (b). The disk works great, can assign a letter to the iSCSI mounted drive on server (b), format it, create files, etc.. Nevertheless, when using it for Windows Backup, It fails to continue once it starts copying files (none are copied indeed). I get "unknown error" with code 0x80042556.
I have been able to make this setup work using a folder on server (a) instead of a USB disk and exporting it through iSCSI. So, it does not seem to be a iSCSI error...
Any hints on where to look for more details on the problem?.