What machine and media are you backing up to? If you're backing up to a FAT32 volume on a Windows machine you may be experiencing the 4Gb file size limit. I had a similar problem backing up to DVD-RAM media as these were FAT32 formatted, a reformat to UDF allowed me to exceed the 4Gb file size limit.
As mentioned I've not had any problems with any of the HP SureStore DAT drives I have around here and even my two DEC DLT drives (a 15/30 and a 35/70) are picked up fine.
Regards,
Jason.