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This is a memo so that you don't forget the operation.
When I used the system I had been using for other purposes as is for Proxmox VE (hereinafter referred to as PVE), the drive connected to it other than the drive where PVE was installed (/dev/sda) was recognized as /dev/sdb, and two Partitions remained. /dev/sdb1 and /dev/sdb2 in the screenshot below↓
I decided to delete these partitions in order to reuse /dev/sdb.
fdisk operation
I didn't know how to do it from the management GUI (web browser), so I used the fdsik command.
《Screenshot》 ↑ At the Command (m for help): prompt, type p. You can also type print. It shows that there are partitions /dev/sdb1 and /dev/sdb2
《Screenshot》 ↑ Enter d to delete the partition. Next, we will delete /dev/sdb1, so enter 1. Next, we will also delete /dev/sdb2, so if you type d, 2 will be automatically selected and partition 2 will be deleted. Type p to confirm that the partition has been deleted. Type w to write changes to the partition table and exit fdsik. The new table will be used at the next reboot. is displayed, indicating that the new partition table information will be reflected by rebooting the system.
Restart PVE system
"screenshot" 
If you check on the PVE management GUI screen, you will see only /dev/sdb. /dev/sdb1 and /dev/sdb2 have been removed.