I believe I have discovered the solution to this problem, thanks to the insightful observation from Thomas Schmitt, whom I warmly thank!
In the BIOS setup utility of ASUS's P5E WS Pro motherboard, under "SATA Configuration," the "Configure SATA as" option can be set to IDE, RAID, or AHCI. At the time I installed the Ubuntu operating system, I left this option as IDE, which is the default setting (perhaps for the benefit of Windows XP users; see below). From the manual: "If you want to use the Serial ATA hard disk drives as Parallel ATA physical storage devices, keep the default setting (IDE)."
To fix the problem of the computer freezing when writing to media using the optical drive, I went into the BIOS and changed the "Configure SATA as" option to AHCI. From the manual: "The AHCI allows the onboard storage driver to enable advanced Serial ATA features that increases storage performance on random workloads by allowing the drive to internally optimize the order of commands." I warn readers that making this change may not be safe, or may not be safe after the operating system has already been installed. An Internet search reveals that Windows users have encountered problems booting their computers after making such a change. I can only testify that on my computer, running Ubuntu 10.04, I had no such problems. It seems that the AHCI drivers are installed by default in Ubuntu 10.04 (and, I would expect, later Ubuntu versions) but not, for example, in Windows XP.
I did encounter two minor problems after making the change. First, after rebooting, an SCSI disk assigned to /dev/sdc was reassigned to /dev/sdd, and vice versa for an SCSI disk that had been assigned to /dev/sdd. As I had previously added lines to my /etc/fstab file providing mount points for /dev/sdc1 and /dev/sdd1, I was temporarily unable to access these drives until after editing my /etc/fstab file appropriately. (This problem probably would not have occurred if I had used UUIDs to identify the devices in /etc/fstab instead of /dev/sd....)
The second minor problem was that I had to reset my Ethernet eth0 connection information following the first reboot after changing IDE to AHCI. In Ubuntu 10.04 this could be done in System -> Preferences -> Network Connections: under IPv4 settings, the address, netmask, and gateway had to be reset to how I previously had them.
Neither of the two minor problems has recurred after multiple subsequent reboots.
After the change from IDE to AHCI, the command "lspci -k" reports for "SATA controller Intel Corporation 82801IR/IO/IH (ICH9R/DO/DH) 6 port SATA AHCI Controller (rev 02)" (previously listed as "IDE interface Intel Corporation 82801IR/IO/IH (ICH9R/DO/DH) 4 port SATA IDE Controller (rev 02)"), "Kernel driver in use" has changed from "ata_piix" to "ahci." The listing for "IDE interface: Marvell Technology Group Ltd. 88SE6145 SATA II PCI-E controller (rev a1)" is unchanged after the switch from IDE to AHCI ("Kernel driver in use" was and is "pata_marvell"). Also, opening System -> Administration -> Disk Utility, I find the hard drives and the optical drive that had been giving me problems are now organized under "SATA Host Adapter" with the 82801IR/IO/IH hardware listing. As I recall, before the change Disk Utility placed them under 82801IR/IO/IH labeled as "PATA Host Adapter."
Since making the change, the command
nice -n 18 ionice -c3 growisofs -dvd-compat -Z /dev/dvd1="video.iso"
does not appear to slow the computer in any perceptible way. I've tried a little bit of benchmarking, such as writing a few megabytes of data to various hard drives after starting the above command and without it with the computer similarly engaged: both wall time and CPU time seem to fall well within the normal range when the command is running (and in particular when it is starting, the problem period before). The problems I had observed immediately after the command when using vim and Firefox do not seem to occur anymore.