Do you know how long it takes to format a 6 TB hard drive over a USB connection?
USB 3.0 6TB HDD I would say between 10 to 14 hours if doing a full format and everything is working optimally.
Best guess for me as there are a lot of factors, the spin speed, actual USB performance, health of the drive etc..
As I recall a reasonable rule of thumb would be 2 hrs per TB for USB 3.0