docs: clarify install disk selection and /dev/sda identity details

Co-authored-by: aider (openrouter/openai/gpt-5.2) <aider@aider.chat>

docs/nixos_build_spec.md

@@ -2,6 +2,7 @@
 
 ## Disk / Boot (DO NOT BRICK WINDOWS)
 Target disk: /dev/nvme0n1
+Install target is NVMe only. Do not select /dev/sdX devices in the installer.
 
 KEEP (do not format):
 - /dev/nvme0n1p1 (EFI vfat) UUID=94CC-9DA4
@@ -13,7 +14,9 @@ WIPE + install NixOS here:
 - /dev/nvme0n1p5 (currently Ubuntu ext4 /) UUID=4e99904c-ce6f-4c3c-898a-e29cfc373489
 
 ## Other attached disks (DO NOT FORMAT)
-- /dev/sda (240GB Patriot Burst) contains NTFS label "Back Up" (keep)
+- /dev/sda (240GB) model: Patriot Burst El (KEEP)
+  - /dev/sda1: Microsoft reserved partition (~15MiB)
+  - /dev/sda2: NTFS label "Back Up" (keep)
 - /dev/sdb (500GB) ext4 label "TempBackup" (keep)
 - /dev/sdc (320GB) ext4 label "Integral300" UUID=27febd74-20aa-4a3a-92c1-6fdd1ad7e88e (mount at /data)
 - /dev/sde (160GB) ext4 label "XPSystemDrive" (keep)