Sandisk iXpand won't record video continuously to drive attached to iPhone 5s with limited memory

SanDisk’s ixpand mini flash drive ads say that customers can “record directly” to the ixpand drive.  No “except if you’re using an iphone.”  SanDisk’s *instructions* also SHOW an upper left icon that states that customers can “record directly” to the ixpand drive.  No “except if you’re using an iphone.”  Please stop stating that claim.  Or else tell us what the secret is to not FIRST have to record to our phone and THEN transfer it to the ixpand drive.  That process, of course, is NOT “directly” recording to the ixpand drive.  I end up getting a “critically low on memory” message, and only getting about a minute of recording time before the recording stops.  NOT what I was buying into when I bought this drive.