I'm working on an embedded project. I need a place to store some filesystem-independent meta information on a storage device. The device has an MSDOS partition table. The device also may have unallocated space (depending on its size) but it will be TRIMmed (and also may be blown away by new partitions in the future).

I need a location on the device that is not unallocated and that has a low risk of being touched (outside of completely erasing the device). The device is only guaranteed to have an MBR at the point the meta data needs to first be written; meaning there are no EBRs/VBRs present that I could use.

There are 446 bytes at the very start of the device available for MBR bootstrap code. Currently my only idea is to store data at the end of this block. However, the device is bootable and I have no way of knowing if I'd be blowing away bootstrap code or not.

The sector size is 512 bytes and the MBR is the first sector, I'm pretty sure (correct me if I'm wrong) that that means the second sector is available for use by partition data, so I can't use that either.

Does anybody have any ideas? I need 4 bytes of space.