automated UM database backup

hello
I want to run this script on my router (RB1100AHx2)

automated UM database backup

:local ts [/system clock get time]
:set ts ([:pick $ts 0 2].[:pick $ts 3 5].[:pick $ts 6 8])
:local ds [/system clock get date]
:set ds ([:pick $ds 7 11].[:pick $ds 0 3].[:pick $ds 4 6])
:local fname ("umdb-".[/system identity get name]."-".$ds.$ts.".backup")

all above is workaround to get unique filename

/tool user-manager database save name=$fname

but When I add the disk1 (name=disk1/$fname) does not run script
what can i do ?
Thanks