From your backup machine, can you START / RUN / "\\servername\drive$" successfully? If not then you have run a script or patch or manually told your remote server not to admin share the drives. Check this setting:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters
AutoShareServer REG_Dword 0x00000001(1) if this is (0) then you have no c$, d$, etc and you cannot touch from remote backup server