The disc I was writing with DirectCD is now unreadable...
1. To screw up DirectCD's UDF handling is to remove the disc without letting the software finish up.
You can accomplish this by rebooting while it's working, attempting to disable it by doing something other than uninstalling it. Also it by turning off auto-insert notification.
2. If you have DirectCD 2.x, you may be able to recover the data with the included Scan Disc utility.
3. CD-R Diagnostic and IsoBuster may also be able to recover data.
Important: Don't delete data off your hard drive until the disc is finalized and verified readable.