Eject an external USB hard drive via the command line
diskpart
list disk:
select disk X
offline disk
exit
This ensures the drive is safely unmounted.

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Pour reformatter un disque dur:
diskpart
list disk
select disk 0
clean : remove partitions
convert mbr : only if required i.e problem installing the OS (convert GPT Disk to MBR Disk)
create partition primary
active
list volume
select volume 1
assign letter=C
exit
then
format c:

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To identify the drives
diskpart
list volume
exit

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keywords: partition disque bios eject ejecter unmount