if i got your question right, you need to make sure that you have an updated slot information for your autoloader when the backup starts?
if you have an autoloader without barcode reader, you cound use omnimm command to do a repositroy Update.
omnimm -repository_update "devicename-from-autoloader"
so if you have a device called "LT5" in your autoloader the command is like
omnimm -repository_update "LTO5"
what this command does, is to do a scan on all slots and so for all media in those slots. If you have an 8 slot autoloader, i would say it takes about 5-10minutes. you could schedule this command before every nights backup to make sure that you have an up to date repository in dp when the backup starts
hope this helps