I'm using QEMU on Linux with KVM, I need to make available a USB drive (external HDD) to the guest while I can still have access to the drive from the host OS. Is this possible?
No, that's not possible. You cannot have access to a usb-device from two different USB-Hosts.
In your case (external drive), you can mount it on the host and then use 9p virtio to forward the mounted folder to the guest or use network-folder share technologies like nfs or smb.