NVME weirdness in BIBM

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CyberSimian
Posts: 137
Joined: Wed Aug 10, 2016 12:13 pm

NVME weirdness in BIBM

Post by CyberSimian »

This looks a bit weird:
embr_partition_table.jpg
embr_partition_table.jpg (174.58 KiB) Viewed 2779 times
Prior experimentation had suggested that this weirdness was related to the EMBR, so I deleted one of the filler partitions, changed the disk type to MBR, and then rebooted BIBM, which gave this correct result:
mbr_partition_table.jpg
mbr_partition_table.jpg (183.21 KiB) Viewed 2779 times
However, after taking the screen shots I changed the disk type back to EMBR and recreated the filler partition. On the next boot, BIBM showed the disk with an EMBR and the correct disk size. So it seems a bit erratic as to whether BIBM shows the correct disk size or an incorrect disk size.

-- from CyberSimian in the UK
TeraByte Support
Posts: 3598
Joined: Thu May 05, 2011 10:37 pm

Re: NVME weirdness in BIBM

Post by TeraByte Support »

It's the BIOS, ensure in LBA mode (some need normal mode) may be in auto mode and ensure you have the "Align MBR for BIOS Auto Mode" otherwise some BIOS will change its reported drive size based on the partition entries ending CHS values.
CyberSimian
Posts: 137
Joined: Wed Aug 10, 2016 12:13 pm

Re: NVME weirdness in BIBM

Post by CyberSimian »

TeraByte Support wrote: Thu Mar 10, 2022 3:46 pm It's the BIOS, ensure in LBA mode (some need normal mode) may be in auto mode and ensure you have the "Align MBR for BIOS Auto Mode" otherwise some BIOS will change its reported drive size based on the partition entries ending CHS values.
In a moment of weakness I purchased a new laptop, specifically this one:
Gigabyte Aorus 15G with Intel i7-10875 and Nvidia RTX-2070 Super
So I am in the process of setting it up in legacy boot mode (it arrived in UEFI boot mode, of course).

Being a laptop, it has very few settings in the BIOS that users can alter. (I believe that there are hidden menus in the BIOS for changing advanced settings, but those menus are inaccessible to users.) In the SATA config I have selected "AHCI" mode (the only alternative is "Intel RST" mode), but I don't think that that is relevant. In NVME config, the only selections are for choosing and running a test on the NVME disk.

In the BIBM settings "Align MBR for BIOS Auto Mode" is already selected. Also selected are "Align on 1 MiB Boundaries" and "Use New Windows MBR". The setting "Validate Geometry Before Use" is selected but greyed out. When I booted BIBM to check these settings, the size of the NVME disk shown in "Partition Work" had reverted to 8 MiB. :(

-- from CyberSimian in the UK
TeraByte Support
Posts: 3598
Joined: Thu May 05, 2011 10:37 pm

Re: NVME weirdness in BIBM

Post by TeraByte Support »

notice the lower left corner of your pics. The CHS values, the ones with C = 0 being reported is the problem, use "View MBR" to look at what has been setup in there.
CyberSimian
Posts: 137
Joined: Wed Aug 10, 2016 12:13 pm

Re: NVME weirdness in BIBM

Post by CyberSimian »

TeraByte Support wrote: Sat Mar 12, 2022 4:21 pm notice the lower left corner of your pics. The CHS values, the ones with C = 0 being reported is the problem, use "View MBR" to look at what has been setup in there.
In experimenting a bit more, I think that I have now identified the circumstances that give rise to the incorrect disk size.

It is my custom to create on every internal disk four filler partitions that are in addition to the data and OS partitions. Each filler partition is 1 MiB in size. When I am installing an operating system, I use these filler partitions to fill the MBR so that there are no unused entries that Windows can use for other partitions (such as a separate Recovery partition). In this way I can force Windows to install everything into a single partition, which makes drive imaging and restoration simpler since the OS image has no dependencies on any other partitions.

If I put the four filler partitions in the boot item for that disk, and then boot Windows (installed on a different disk), when I next restart the system and go to "Partition Work", BIBM shows the incorrect 8 MiB size for the disk. As you noticed, the "C" value for the disk is shown as 0, but the values in the MBR look reasonable (at least, to me). All of the filler partitions begin on C=1023, but have different and believable starting values for H, S, and LBA. However...

If I put the data partition plus three filler partitions in the boot item, and then boot Windows, when I next restart the system "Partition Work" shows the correct size for the disk. This is why initially the occurrence of the incorrect disk size seemed erratic -- I had not realised that it was correlated with what I had booted previously.

I don't know whether this strange behaviour is explicable, or whether it could be avoided by placing the filler partitions at the beginning of the disk instead of at the end. (I have some data in the data partition, so I don't really want to try that experiment at the moment.) In everyday use, I will always have the data partition in the boot item, so this strange behaviour will not be apparent and will (hopefully) be inconsequential.

Thank you for the time you spent looking at this.

-- from CyberSimian in the UK
TeraByte Support
Posts: 3598
Joined: Thu May 05, 2011 10:37 pm

Re: NVME weirdness in BIBM

Post by TeraByte Support »

yes, my guess is the programmer of the bios is looking that the chs entries and misconfigurating the chs values. That method was something that came out something like 15 to 20 years ago if I can recall.
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