This time the firmware updated .... only ... my folder full of ALL my client files is GONE. The folder is there. It is just empty.
Firmware update will not affect files on Time Capsule disk or USB disk plugged into an Extreme.
If the later you can easily plug it into a computer and see if the files are really gone.. or it is merely Sierra / High Sierra.. playing funny permissions games which is now COMMON.
If this is a time capsule and you are running Sierra or High Sierra on the computer.. it is much more complicated.. you need to pull the drive from the TC and use a SATA to USB adapter or enclosure and then plug it into the computer..
However I did find a fix if it is merely permissions.
Tell me exactly what you are doing.
What airport.. actual model number?
If USB plugged into Extreme.. have you plugged the USB into the computer??
What OS is on the computer?
How did you create the folder? And how long have the files been sitting on it?
How are you protecting the files on the Airport? Do you have device or disk password?? Or much worse username and passwords for users?
Why don't you have backups for client files to say nothing of your own?? (you don't have to answer this.. but you should rap yourself across the knuckles)