The problem is that inventory items in Netbox do not exist as standalone items.
- you cannot remove an inventory item from a device and retain it (you can only delete it)
- you cannot move an inventory item from one device to another device; you can only delete it from A and recreate it in B
- you cannot have a spare inventory item, i.e. one which is not associated with a device
- you cannot track inventory items as long-lived assets: e.g. date purchased, date warranty expires, history of removal between devices, history of return to vendor for refurbishment etc.
At the moment, your best option might be to maintain a separate database or spreadsheet of disks, and just use the inventory items feature in Netbox as a pointer to an entry in that database, indicating where the item is currently installed.