Understanding the conversion from a GPT partitioned disk into a MBR partitioned disk
This conversion can create difficulties arising out of the differences in the on-disk format of the two partition table structures.
The term LBA describes an access method to a disk by using a rising "house number" starting at number zero, ending at n-1 where n denotes the total number of sectors of the disk.
GPT (stands for GUID Partition Table)
GPT is a simple structure starting at LBA=0 consisting of a header starting at LBA=1 and a following table where each entry is pointing to a start and end sector of a partition in terms of LBA (logical block address). There is a backup of the structure at the end of the disk.
To prevent legacy operating systems from overwriting this space not recognizing the GPT structure a protective MBR is placed in LBA=0 telling a legacy operating system that the whole space is already used up.
So the on-disk structure would look like:
pMBR GPTHeaderAndTable p1 p2 p3 p4 p5 p6 ... GPTbackup
There is no requirement of free space between each of the partitions.
MBR (stands for master boot record)
MBR initially starts as a simple table as well, located in the first sector of a disk (LBA=0), containing a maximum of four entries. Later on Microsoft extended this structure by introducing the extended partition. An unlimited number of so-called logical partitions can be held in this structure.
The disadvantage is its on-disk layout: It requires a chained list of additional partition tables residing between the space occupied by each partition.
A simple MBR in LBA=0 could hold just a primary partition entry.
When dealing with two partitions, a primary partition and another logical one, a second partition table was required residing in between of the primary partition and the logical one:
MBR primary1 ext_table logical1
Note that the second partition labeled
logical1 does not follow immediately
ext_table uses up one sector.
When dealing with three partitions, a primary one and two logical ones the MBR structure would require the MBR at LBA=0, and one partition table between each of the logical ones:
MBR primary1 ext_table logical1 ext_table logical2
ext_table uses up at least one sector.
Creating a MBR in Microsoft style
To make things worse, Microsofts fdisk was initially placing partition tables on cylinder boundaries [cylinder=anything, head=0, sector=1] and the partition itself was placed in [cylinder=anything, head=1, sector=1]
On a disk with a CHS-layout of 63 sectors that would yield in 63 unused sectors.
CHS-adressing (cylinder, head, sector) is an outdated scheme to adress sectors. It's information is required for legacy operating systems such as Windows XP. MBR-style partition tables contain CHS-style and LBA-style information to address partition.
Depending on how compatible your conversion program wants to be, it needs at least 1 sector for an additional partition table between each partition.
With the appearance of advanced disk format and its physical sector size of 4096 bytes it was pretty dumb (perfomance-degrading and stressing for the disk) to follow those practices. I think Gparted at least and maybe others allowed to partition differently, putting partition starts on megabyte boundaries (wasting 2048 sectors between each partition (2048*512=1048576)).
The space requirement of a MBR-style partition could cause the move of a partition by some sectors. This is a risky operation especially when there is no power buffer like a laptop battery that suggest making a backup. With the time needed for a backup one might consider rather getting a second disk partition the disk and copy the partitions over to the new disk one by one.
comments regarding your comments
There is no guarantee of this conversion to work and it may be a risky operation. The above should explain that to you.
I try to explain additional space requirements. On the other hand, the space for the GPT table and its backup is not required anymore. But the conversion method can increase space requirements the more partitions you have.
PLease don't use abbreviations like ELI5. I am not a native English speaker and I have to look them up. Thank you.