DLT 7000 tape drive failing with "write error on a record in the index"

HP rp5450 (L2000)
running HP-UX 11.11B
Using DLT 7000 and DLT 4000 tape drives for nightly full backups
Backup jobs created by SAM

DLT 7000 cron entry is as follows:
00 2 * * 1-6 /usr/sam/lbin/br_backup DLT FULL Y /dev/rmt/0m /var/sam/graphLCAa17036 root Y 1 N > /var/sam/SAM_br_msgs 2>&1 #sambackup

On the DLT 7000 have been getting failures for the past month or so, most of which are "write error on a record in the index".

Have tried swapping tape drive positions in the enclosure (SCSI bus) with no luck.
Have tried different tape manufacturers & new/old tapes with no luck.
Have tried multiple DLT 7000 drives with no luck.
Backup jobs using to DLT 4000 are working successfully for same dataset.

Any ideas as to what is causing this failure & how to resolve?

Thanks

I see that you have already posted on the HP ITRC, which would have been my advice.

The general consensus in this old ITRC post is that the K Class server can handle two DLT4000 drives daily-chained, but if you have a DLT7000 it must not be daisy-chained. Whilst not an exact match to your problem it should give some clues.
HP Communities - fbackup and dlt tape drives in K-class server - Enterprise Business Community

Have you checked for end-of-life on your Server and its system software?

DYOR and caveat emptor.