strapper | simple tool to help bootstrap necessary resources

There are a number of problems with your code, but it may well be that the issue is in the volume boot record you didn't show. Some of the problems in the MBR that should be resolved:

• Set SP right after SS to ensure that an interrupt can't occur between the setting of SS and SP, which would corrupt memory at the address formed by the unknown old SP and the new SS (or vice versa). The CPU automatically turns off interrupts after setting SS and re-enables them after the following instruction.
• Issue a CLD instruction to clear the direction flag (DF) so that string instructions like MOVSB and LODSB use forward movement.
• When using Int 13h/ah=42h there are some BIOSes that need ES:BX set to the same values in the Disk Address Packet (DAP). Your code improperly sets ES. It should have been set to zero.
• When filling in the start LBA in the DAP from the value in the active partition entry your code only copies the lower 16 bits of a 32-bit value. This limits you to media that is <= 32MiB (512*65536). You should copy both the lower and higher half of the starting LBA from the partition table to the DAP.
• When doing a disk read or write you should retry the operation 3 more times before failing. This may be required on real hardware using actual floppy and hard drives.
• You should check for an active partition by checking that the value is 0x80 and not just the top bit. The only valid values are either 0x00 or 0x80.
• The volume boot record (VBR) you are loading from the partition is usually one sector. If it is one sector, read one sector and not the whole partition.
• Your code is overly complex in the way you set up the sections. If you are going to relocate the bootloader to 0x0600 then use an ORG of 0x0600. Just make sure that the code that relocates the boot sector from 0x0000:0x7c00 to 0x0000:0x0600 doesn't rely on any labels that are relative to 0x7c00.

There are some things in your code that are nice to have:

• Clean up how you loop through the partition table searching for the active/bootable partition.
• If you ever wanted to remain compatible with some ancient OSes then pass the address of the bootable partition entry in DS:SI before jumping to the volume boot record you read into memory. This isn't required
• Not a requirement and this is just meant as a note: if you ever wanted to retain compatibility with MS-DOS, partitions on a hard drive should always be on a cylinder boundary and preferably end on a cylinder boundary.

Some of these tips can be found in my Stackoverflow General Bootloader Tips.

A modified version of your relocatable bootloader that chain loads a Volume Boot Record (VBR) could coded as:

boot.asm:

If I understand it correctly, you want to remove all the tags. You can do this with .extract() [bs4-doc]: