The "disk I/O error" part suggests you have a problem with some of the sectors on the drive.
Using the windows diagnostic tools, run some checks on that drive.
Thanks for the advice on using the Windows Diagnostic Tools on the drive. When we backup the lightroom catalog, Lightroom says it is "checking the integrity of the G Drive", then it backs up the catalog. Doesn't that do the same thing as using the Windows Diag. Tool? Since it does back up, wouldn't that mean the G drive, and its sectors, are okay?
Thank you very much for your help.
No, what LR is going is making sure that the drive is available and that it has enough space on it.
You need to do a low level analysis on the drive using the Windows disk tools. It will take a very long time to complete.
In Windows File Explorer, right-click on the drive, select "properties", tools, "check".