how do I avoid buffer underrun?

I'm trying to write a dual-layer blu-ray using the schily cdrecord
Cdrecord-ProDVD-ProBD-Clone 3.01a06 (x86_64-unknown-linux-gnu)
Copyright (C) 1995-2011 Joerg Schilling

I can create an ISO image that's 38,961,047,552 bytes, which should
fit on a 50G dual-layer blu-ray disk.
I am having trouble writing the image onto the disk. I get a message
about buffer underruns, and need some help fixing this.

Thanks for any help you can give,
Chelmite

I created the iso image with:
% mkisofs -o /media/Maxtor2/Backups/maxtor3.iso -r -J -hide-rr-moved -
V "Backup 2011/11/20" -publisher "Chelmite" -iso-level 4 -graft-points
photos/M3/Photos=/media/M3/Photos photos/M3/JDroid=/media/M3/Backups/
Droid/JDroid/dcim photos/M3/TDK-Vibrant=/media/M3/Backups/Droid/
JVibrant/DCIM photos/M3/SDroid=/media/M3/Backups/Droid/SDroid/dcim
photos/M3/SVibrant-KD=/media/M3/Backups/Droid/SVibrant1/KDROID32/dcim
photos/Maxtor3/SVibrant-SD=/media/M3/Backups/Droid/SVibrant1/SDROID/
DCIM
Warning: Creating ISO-9660:1999 (version 2) filesystem.
Warning: ISO-9660 filenames longer than 31 may cause buffer overflows
in the OS.

Then I tried to write the iso:
% /opt/schily/bin/cdrecord tsize=19023949s -dao fs=12m speed=-1
driveropts=burnfree dev=/dev/sr1 - /media/Maxtor2/Backups/
maxtor3.iso
Cdrecord-ProDVD-ProBD-Clone 3.01a06 (x86_64-unknown-linux-gnu)
Copyright (C) 1995-2011 Joerg Schilling
scsidev: '/dev/sr1'
devname: '/dev/sr1'
scsibus: -2 target: -2 lun: -2
Warning: Open by 'devname' is unintentional and not supported.
Linux sg driver version: 3.5.27
Using libscg version 'schily-0.9'.
Device type : Removable CD-ROM
Version : 0
Response Format: 2
Capabilities :
Vendor_info : 'PIONEER '
Identifikation : 'BD-RW BDR-206D'
Revision : '1.56'
Device seems to be: Generic mmc2 DVD-R/DVD-RW/DVD-RAM.
Using generic SCSI-3/mmc-3 BD-R driver (mmc_bdr).
Driver flags : NO-CD BD MMC-3 BURNFREE
Supported modes: PACKET SAO LAYER_JUMP
cdrecord: Warning: Cannot read drive buffer.
cdrecord: Warning: The DMA speed test has been skipped.
WARNING: Phys disk size 4452354 differs from rzone size 24438784!
Prerecorded disk?
WARNING: Phys start: 25088 Phys end 4477441
Starting to write CD/DVD/BD at speed 8 in real SAO mode for single
session.
Last chance to quit, starting real write 0 seconds. Operation
starts.
cdrecord: Input/output error. write_g1: scsi sendcmd: no error
CDB: 2A 00 00 BA 74 10 00 00 10 00
status: 0x2 (CHECK CONDITION)
Sense Bytes: 70 00 05 00 00 00 00 0E 00 00 00 00 21 02 00 00
Sense Key: 0x5 Illegal Request, Segment 0
Sense Code: 0x21 Qual 0x02 (invalid address for write) Fru 0x0
Sense flags: Blk 0 (not valid)
resid: 32768
cmd finished after 13.786s timeout 200s
write track data: error after 25025347584 bytes
cdrecord: The current problem looks like a buffer underrun.
cdrecord: It looks like 'driveropts=burnfree' does not work for this
drive.
cdrecord: Please report.
cdrecord: Make sure that you are root, enable DMA and check your HW/OS
set up.

1. I ran this command without driveropts=burnfree, and got a message
saying I should use that same burnfree option:
cmd finished after 11.627s timeout 200s
write track data: error after 25,025,347,584 bytes
cdrecord: The current problem looks like a buffer underrun.
cdrecord: Try to use 'driveropts=burnfree'.
cdrecord: Make sure that you are root, enable DMA and check your HW/OS
set up.

2. I ran the cdrecord command as root both times, yet cdrecord warned
that I should be root.
3. I don't know how to enable DMA. I do have AHCI enabled in the BIOS.
4. My data source is connected to my computer via eSATA.
5. My Blu-ray writer is connected at the slower, USB-2.0 port, to
avoid buffer underruns.
6. Pioneer says the Blu-ray writer is supposed to work at 12x speed,
and has a 4M buffer. My laptop has 4G RAM, 0.25G free.