A backup medium (SSD or whatever) which only allows writes to empty space. Unless a switch is manually switched from "write" to "update".
In "write" mode, it would only allow writing to empty space.
In "update" mode, it would allow writing everywhere.
I would leave it in "write" mode most of the time. For me, a typical SSD has enough space for years of incremental backups. If I should ever want to delete old backups, I would set it to "update" mode to do that and then set it back to "write" mode.
What I would trust:
A backup medium (SSD or whatever) which only allows writes to empty space. Unless a switch is manually switched from "write" to "update".
In "write" mode, it would only allow writing to empty space.
In "update" mode, it would allow writing everywhere.
I would leave it in "write" mode most of the time. For me, a typical SSD has enough space for years of incremental backups. If I should ever want to delete old backups, I would set it to "update" mode to do that and then set it back to "write" mode.