A A measure of how quickly large files can be written to a device.Sustained sequential write speed is calculated as the average Sequential write speed over a 60 second period. This helps to measure real world steady state speed rather than SLC or RAM cached performance.
The exact amount of space used depends on the specific drive but it is typically:
USB @ 50 MB/s for 60s = 3GB file size
HDD @ 150 MB/s for 60s = 9GB file size
SSD SATA @ 400 MB/s for 60s = 24GB file size
SSD NVMe @ 1500 MB/s for 60s = 90GB file size
In order for the test to complete there has to be enough free space on the drive.