My internal DVD drive broke on my MacBook, so I replaced it with an external SATA Sony Optiarc AD-7240S connected to a USB port using a cheap ESATA-to-USB dongle. Everything works great, burns are fast, etc. However, DVD Player in OS X 10.6.2 errored out when trying to play a DVD (VLC played it just fine). The message from DVD Player was "a valid drive could not be found error -70012."
After some net searching, I found the solution here and here. Important note! If you have an internal DVD burner, that drive will no longer be supported by DVD Player after this modification!
You'll need Administrator access priviledges. Navigate into /System » Library » Frameworks » DVDPlayback.framework » Versions » A. Make a backup and a copy (i.e two different copies) of DVDPlayback on your Desktop.
Open the copy with a hex editor such as 0xED. Find the word Internal and replace with External by searching for (hex) 496E7465726E616C and replacing with 45787465726E616C. There will be about four to six matches that need replacing. Save the file and replace the original with your edited version. Make sure you keep your backup file, in case you need to revert your changes.
Repair permissions and unplug your external DVD burner, then restart. DVD Player will now work with the external drive.
[robg adds: I haven't tested this one, for somewhat obvious reasons. If you're going to try it, definitely keep a backup, and proceed at your own risk.]
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