Hi,
You have a solution for. But your USB disk cannot be umounted of your Proxmox server after this kind of modification.
In WebUI, click on your PRX host. In the middle column, click on "disks".
Identify the name of your USB disk partition( /dev/sdXy)
If you have data on your disk, you need to know his storage type (xfs, ext4,ntfs, ...)
In the middle column again, click on "shell".
go to /mnt
Create a mountpoint for your USB disk (exemple /mnt/usb-storage)
Modify your /etc/fstab file. You need to add after all present line this:
/dev/sdXy /mnt/YOUR-MOUNTPOINT ext4(orwhatever you use) defaults 0 0
Save your file.
type the command: mount -a
Now in the first column of the WebUI, click on "datacenter".
2nd column: storage
click on "add", "repository"
Enter a name. Give the directory (in the example /mnt/YOUR-MOUNTPOINT) .
Save.
Now, you have a new datastore called by the name given just before. Go to the VM configuration, and add a new Virtual disk stored in the usb-datastore.
Best regards