`I recently purchased a refurbished 1Dx. The camera produces files named "FUST" instead of "IMG".

[hsbn]

I don't have a 1DX but if I'm not mistaken, you can change the file name in that camera. Maybe previous owner has changed it. You'll need to change it back. You can try custom function reset but I'm not sure if you can reset it (some functions doesn't get reset).

[Mykolas · Accepted solution]

Hi EastLelandDavid!

The file name on the EOS 1D X has four alphanumeric characters followed by a four-digit image number and extension.  The first four characters are set upon factory shipment and unique to the camera.  However, you can change them.  With [USER SETTING 1], you can change and register the four characters as desired.  With [USER SETTING 2], if you register three characters, the fourth character from the left will be appended automatically to indicate the image size.  Please follow these steps:

1. Press the MENU button
2. Select the [Setup #1] tab.
3. Select [File Name].
4. Select [Change User Setting 1] or [Change User Setting 2].
5. Enter any alphanumeric characters.

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[EastLelandDavid]

Thanx much! That was exactly for what I was looking. Problem resolved. Thanx again.
David