[cesarb]

> After freeing the new SATA SSD, I also filled it with butter. Yes, all the way, no GPT, no MBR, just Btrfs, whose subvolumes were used in place of partitions

I would not recommend doing that. It might work for now, but there's a high risk of the disk being seen as "empty" (since it has no partition table) by some tool (or even parts of the motherboard firmware), which could lead to data loss. Having an MBR, either the traditional MBR or the "protective MBR" used by GPT, prevents that, since tools which do not understand that particular partition scheme or filesystem would then treat the disk as containing data of an unknown type, instead of being completely empty; and the cost is just a couple of megabytes of wasted disk space, which is a trivial amount at current disk sizes (and btrfs itself probably "wastes" more than that in space reserved for its data structures). Nowadays, I always use GPT, both because of its extra resilience (GPT has a backup copy at the end of the disk) and the MBR limits (both on partition size and the number of possible partition types).

[LanternLight83]

Neat! I went looking for ways to create a protective MBR, learned a lot from the Gentoo wiki, some interesting info about how Windows does things in the link below, but the way to achieve this seem to be to just format the disk as GPT and then truncate it to the MBR (or just use one big GPT partition).

https://thestarman.pcministry.com/asm/mbr/GPT.htm

[cesarb]

I would also not recommend formatting the disk as GPT and "truncating it to the protective MBR". Not only there's a good chance of the GPT re-appearing on its own, either because some software noticed it was corrupted and copied it from the backup copy at the end of the disk, or because some software noticed it was missing and created a new one, but also there's a chance of it once again being treated as if the whole disk was empty (since the "protective MBR" says it's a GPT disk, and the GPT has no entries). If you want to have just the single MBR sector, then create a traditional MBR with a single partition spanning the whole disk instead of a GPT "protective MBR". But that will not gain much, since you should align your partitions (IIRC, usually to multiples of 1 megabyte) for performance and reliability reasons (not as important on an HDD, where you can align to just 4096 bytes or even 512 bytes depending on the HDD model, but very important on an SSD), and the space "wasted" by that alignment is more than enough to fit the GPT.

[LanternLight83]

Ah, I missed there was a unique label indication GPT was involved in the protective MBR; I I'd read it, it essentially is just an MBR with a single max-size entry, and I didn't consider there might be anything else; and ofc, it is a bit of a pointless thought experiment. Thanks for sharing!