Location on disk to write a byte-flag?

Question

I am writing an update utility for an embedded Linux device for which I am responsible.

I'm creating a basic side-by-side setup, where updates are applied to a staging partition and then the bootloader is switched to the staging partition, changing the primary partition into the new staging one.

However, the embedded device uses a CF card, which are known for sometimes writing data out of order, etc. For this reason, if I mount a file system as read/write, there's a chance of corruption. Therefore, I cannot mount my bootloader partition RW in order to point to the new partition.

Is there a location on disk to which I can safely write a single byte, where the byte represents a 0 for the first partition or a 1 for the second? Even a CF card can't screw up a single byte write, which should happen atomically.

I'm using a plain-old DOS MBR format disk.

Answer 1

Create a third, tiny, partition to hold your data. Any other location on your disk will sooner or later bring trouble if indeed you cannot rely on the filesytems.

Sometimes the last few clusters of a disk cannot be addressed in the FAT entry, that may be an option but it heavily depends on exact size of the device.

Does the embedded CPU / device have EEPROM? That would be an ideal place for a single byte.