I/O errors when running "head", UNLESS CD is already spinning - how to unload cache?

[FlameWeasel]

Ok, this is a weird one.

I've burned at least five DVDs and/or CDs with 6.0.2, and every single one of them does the same thing.

If I try to run the program "head" (as in, "head -5 stuff.txt", to display the first five lines of the file), and if the DVD or CD isn't already spinning, I get the error, "bash: /usr/bin/head: Input/output error". Moreover, the error is permanent until I reboot -- the error is somehow cached, so that the system *knows* that the optical disk must be hosed, and doesn't bother to retry.

If, on the other hand, I type in a few other commands, doesn't really matter what they are as long as it gets the CD/DVD spinning, then trying to run "head" works just fine and dandy. How do I get the system to un-cache the "disk is hosed" error so that I can actually run the command without having to reboot?

Note that similar problems happen when I put the system into sleep mode and then re-wake it, but they happen on EVERY command, so finding out how to clear out this cache may help prevent more problems.