Windows Server 2019 convert from EFI to legacy/BIOS ?

[nade]

Just wondering if anyone has had any success (going backward) and changing a built server to use legacy BIOS?

From what I've been able to dig up online, the general view is it's not that simple. Either a full wipe, restore from backup is needed. Or replacing the OS disk with a new disk, copying the boot partition over and running a fix of the MBR.

At this stage it's probably less time consuming for the server to be replaced/rebuilt.

 

[andshrew]

I don't think there is a Windows tool for doing this.

If you can boot a Linux live CD you may be able to use gdisk with commands:

x (for extra functionality)
r (for recovery and transformation)
g (for convert GPT to MBR)
w (for write the MBR partition table and exit).

You would then reboot and need to use a Windows installer to re-create the MBR boot files for Windows.

But in honestly, it's probably easier rebuild.

Spoiler: Example output from gdisk

user@linux:~$ sudo gdisk       
GPT fdisk (gdisk) version 1.0.8

Type device filename, or press <Enter> to exit: /dev/sda
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): ?
b    back up GPT data to a file
c    change a partition's name
d    delete a partition
i    show detailed information on a partition
l    list known partition types
n    add a new partition
o    create a new empty GUID partition table (GPT)
p    print the partition table
q    quit without saving changes
r    recovery and transformation options (experts only)
s    sort partitions
t    change a partition's type code
v    verify disk
w    write table to disk and exit
x    extra functionality (experts only)
?    print this menu


Command (? for help): x

Expert command (? for help): ?
a    set attributes
b    byte-swap a partition's name
c    change partition GUID
d    display the sector alignment value
e    relocate backup data structures to the end of the disk
f    randomize disk and partition unique GUIDs
g    change disk GUID
h    recompute CHS values in protective/hybrid MBR
i    show detailed information on a partition
j    move the main partition table
l    set the sector alignment value
m    return to main menu
n    create a new protective MBR
o    print protective MBR data
p    print the partition table
q    quit without saving changes
r    recovery and transformation options (experts only)
s    resize partition table
t    transpose two partition table entries
u    replicate partition table on new device
v    verify disk
w    write table to disk and exit
z    zap (destroy) GPT data structures and exit
?    print this menu

Expert command (? for help): r

Recovery/transformation command (? for help): ?
b    use backup GPT header (rebuilding main)
c    load backup partition table from disk (rebuilding main)
d    use main GPT header (rebuilding backup)
e    load main partition table from disk (rebuilding backup)
f    load MBR and build fresh GPT from it
g    convert GPT into MBR and exit
h    make hybrid MBR
i    show detailed information on a partition
l    load partition data from a backup file
m    return to main menu
o    print protective MBR data
p    print the partition table
q    quit without saving changes
t    transform BSD disklabel partition
v    verify disk
w    write table to disk and exit
x    extra functionality (experts only)
?    print this menu

Recovery/transformation command (? for help): g

MBR command (? for help): ?
a    toggle the active/boot flag
c    recompute all CHS values
l    set partition as logical
o    omit partition
p    print the MBR partition table
q    quit without saving changes
r    set partition as primary
s    sort MBR partitions
t    change partition type code
w    write the MBR partition table to disk and exit

MBR command (? for help): p

** NOTE: Partition numbers do NOT indicate final primary/logical status,
** unlike in most MBR partitioning tools!

** Extended partitions are not displayed, but will be generated as required.

Disk size is 125829120 sectors (60.0 GiB)
MBR disk identifier: 0x00000000
MBR partitions:

                                                   Can Be   Can Be
Number  Boot  Start Sector   End Sector   Status   Logical  Primary   Code
   1                  2048      1050623   primary     Y        Y      0xEF
   2               1050624    125827071   primary              Y      0x83

MBR command (? for help): w

Converted 2 partitions. Finalize and exit? (Y/N): y
Warning: The kernel is still using the old partition table.
The new table will be used at the next reboot or after you
run partprobe(8) or kpartx(8)
GPT data structures destroyed! You may now partition the disk using fdisk or
other utilities.