Fixdisk repeatedly unable to find partition 1

That output represents the partition scheme that is stored in the MBR (label: dos means the partition structure is MBR). If that output is given to sfdisk, the partition scheme will be re-created, including writing an equivalent MBR in the first sector (note that the first partition starts at sector 8) [re-edit].
 
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Does this mean that your command

PARTDIR=/var/lib/humaxtv_backup/partition
mkdir -p "$PARTDIR" && /sbin/sfdisk /dev/sda --dump >"$PARTDIR/sda.dump"

will backup MBR somewhere on the hard drive?

If the drive doesn't work like in the OP case does your second command still restore the mbr?
 
Obvs not on the hard drive as it wouldn't be accessible if the drive's partition table got trashed (among other possible failures).

/var/lib/humaxtv_backup/partition is (or will be made by mkdir -p) a directory in the system's flash storage (/dev/mtdblock2), like the storage in a USB stick but soldered into the settop box's circuit board.

In the command that you quote, you can replace /sbin/sfdisk /dev/sda --dump with the complicated command that you ran. Then the output will be saved and could potentially be used to restore the MBR.

A paranoid user could also copy the saved data to an off-line backup, say an actual USB stick.
 
When a disk is formatted with the GPT format by the telnet menu there is a two stage process in creating the GPT and then a non-standard MBR which is recognisable to the Humax app. (see /bin/tmenu).

Will the sfdisk dump command above, save the MBR and the GPT in this situation?
 
Good point. You'd hope so, since sfdisk is the program used in /bin/tmenu to create the dummy MBR and the CF version is supposed to handle GPT and MBR disks, but obviously a test would give confidence. From man sfdisk I suspect that this hope is misplaced, as the defined input format doesn't mention any way of specifying a nested label (aka hybrid MBR). And the confirmation is here.

Apparently we would need to save two dumps and then restore them in reverse order.
 
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