Backing up to an external hard disk drive.

Backing up to an external hard disk drive.

Windows backup tells me that my target drive has 379.48GB free space:

It tells me that my back-up this time will occupy 77.21GB.

Yet it tells me that the backup cannot be run because my target disk does not have enough space.

What is going on here?  Have I misunderstood the measures of size/capacity?