You are only backing up 153 GB. Dependeing on what you are backing up the backup file is only going to be 20-30GB. So there is more than amble space. Backing up with the internal HDs is always faster so this procedure is preferable even if USB to USB worked.

You can do incrementals even if you move the original backup.  Acronis always asks where the backup file(s) is. But the original problem that Acronis don't want to copy from USB to USB may still persist.