For the 5D, it depends upon the size of the memory cards you are going to use. As I remember, you are limited to 8gb cards with 1.0.1
Here's the important caveat from another Canon forum site:
If the newer FW doesn't have a specific feature or upgrade that you need, you do not need to do it. As a matter of fact if you have a perfectly running 5D, I would not under any circumstances chance a FW upgrade. On the chance, albeit slim chance, it goes wrong you will have a brick and Canon can no longer unbrick it for you.
So I would recommend the same advice unless you need to use a larger memory card.
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So, in case you do want to move forward with a Firmware update:
I would add that unless you have some experience performing a firmware update, really take care and read how to do it before you try. It's not hard, but there are a couple of important steps on all updates.
You can Google it and you will get a number of websites. I just tried one (
https://eoscard.pel.hu/) and successfully created the FW 1.1.1 fir file. Let me know if you'd like me to send the fir file to you. I have not used the file, so I take no responsibility for it. I just created it from the download site.
Let me know...
Glenn
For the 5D, it depends upon the size of the memory... (