You may have seen that the EOS R5 is rather fussy about CFExpress cards. Well I can confirm this is the case.

The SanDisk Extreme PRO 512GB card is on the supported list, but my R5 refused to recognise it, even after formatting in MacOSX and Linux. I raised a support call - who suggested that I made a 'long format' on Mac OSX.

However, since Big Sur (and perhaps before), the longer option for Solid state devices have been removed.

So I used the following to write zeros at every position on the card:

sudo dd if=/dev/zero of=/dev/disk3
dd: /dev/disk3: end of device
1000215217+0 records in
1000215216+0 records out
512110190592 bytes transferred in 20238.107872 secs (25304252 bytes/sec)

As you can see this took nearly six hours! It worked, the card is recognised by the camera and I can now use video with the R5.

If you need to use this yourself ... you need to be ABSOLUTELY sure that the /of=your_card_disk_device is correct - otherwise you run the risk of unrecoverable wiping of one of your disks.

You may need to unmount the disk first, I used:

sudo diskutil unmount /dev/disk3s1 <- this is the formatted partition on the disk.
- Volume R5EXFAT on disk3s1 unmounted