There are known problems with Canon and some cards.

http://www.dpreview.com/news/0505/05...lexaralert.asp
Both Canon and Lexar have issued media alerts which describe issues discovered between several Canon digital SLRs and Lexar Pro 80x CompactFlash cards.

That's back in May 2005 and I have one of the problem cards.
It worked OK in my 20D but failed in both the 1D bodies.