What does the color of the bar in Disk Utility mean?

Using Disk Utility, I ran a check on my main HD and my three external disks.

The S.M.A.R.T. status of my main HD was "verified". The external drives were "Not Supported". I assume that means that Dick Utility is unable to "verify" them.

What does concern me is the color of the bar above the chart. On my main HD and two of my external drives, it's blue (see pic#1). However, on one external drive, it's red (see pic #2).

I can find no information about the color of the bar in Apple Help. The drive has a bit over 33% available strength and appears to be working properly in regard to

accessing media. Do you have any idea what this could mean?

Thanks,

Philip