gparted and parted not recognizing partitions in "non-standard" MBRs with valid partition tables
While parted 2.3 had no issues with whatever was in the first 446 bytes of the MBR and only looked at the partition table, newer versions such as 3.2 seem to be thrown by unusual boot code (e.g. Grub4DOS) and either show the entire disk as unallocated space or as one big volume/partition with the file system of the first volume. This is despite the partition table being fully compliant and happily read by fdisk and sfdisk.
This may be a "feature" rather than a bug (required for GPT?). However it could save someone many hours of frustration if this was noted prominently in the documentation (maybe here: https://gparted.org/h2-fix-msdos-pt.php)
It was fixed with a simple:
dd if=/usr/lib/syslinux/mbr/mbr.bin of=/dev/sdb bs=446 count=1
after taking the requisite backups and a deep breath.