try adding disk

system-config/configuration/homebox/default.nix

@@ -7,7 +7,7 @@
 
       inputs.disko.nixosModules.default
       
-      (import ./disko.nix { device1 = "/dev/nvme0n1"; device2 = "/dev/nvme1n1"; })
+      (import ./disko.nix { device1 = "/dev/nvme0n1"; device2 = "/dev/nvme1n1"; device3 = "/dev/sda"; })
       
       inputs.impermanence.nixosModules.impermanence
 

system-config/configuration/homebox/disko.nix

@@ -1,6 +1,7 @@
 {
   device1 ? throw "Set this to your disk device, e.g. /dev/sda",
   device2,
+  device3,
   ...
 }: {
   disko.devices = {
@@ -61,6 +62,23 @@
                   };
               };
           };
+          hdd1 = {
+              device = device3;
+              type = "disk";
+              content = {
+                  type = "gpt";
+                  partitions = {
+                      ssd1 = {
+                          name = "hdd1";
+                          size = "100%";
+                          content = {
+                              type = "lvm_pv";
+                              vg = "hdd1_vg";
+                          };
+                      };
+                  };
+              };
+          };
       };
       lvm_vg = {
           root_vg = {
@@ -109,6 +127,24 @@
                   };
               };
           };
+          hdd1_vg = {
+              type = "lvm_vg";
+              lvs = {
+                  ssd1 = {
+                      size = "100%FREE";
+                      content = {
+                          type = "btrfs";
+                          extraArgs = [ "-f" ];
+                          subvolumes = {
+                              "/hdd1" = {
+                                  mountOptions = [ "subvol=hdd1" "noatime" ];
+                                  mountpoint = "/hdd1";
+                              };
+                          };
+                      };
+                  };
+              };
+          };
       };    
   };
 }