5.42 TiB = 6 TB.
It's Base-2 math versus Base-10 Math.
From a bazillion different WD web specification pages:
"As used for storage capacity, one megabyte (MB) = one million bytes, one gigabyte (GB) = one billion bytes, and one terabyte (TB) = one trillion bytes. ... etc "
So the DUO has 6 trillion bytes total, and the operating system software uses some of that.
The OS on your PC is actually reporting "Tebibytes," not "Terabytes," hence the difference in calculation. Your PC is reporting the size using the wrong abbreviation. It should say 5.4 TiB.
But I'm surprised your PC is reporting 5.6 TB available... It should say closer to 5.42 TiB.