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I am connecting a 4-bay HDD hub (FANTEC QB-35US3-6G) via USB to my Raspberry Pi. I have two disks inside the hub and formated them as FAT.

The formating I did on a Mac because I was not able to see the unformated disks in the hub with blkid when connected to the Raspberry, which is strange.

When sudo blkid I see

/dev/sdc1: LABEL_FATBOOT="EFI" LABEL="EFI" UUID="67E3-17ED" TYPE="vfat" PARTLABEL="EFI System Partition" PARTUUID="e36842bb-f2a9-4a3e-99b6-bbd4a54f39f6"
/dev/sdc2: LABEL_FATBOOT="WD3" LABEL="WD3" UUID="4568-1704" TYPE="vfat" PARTUUID="576db57a-0543-4f9b-b3e4-4cf452cbdda3"
/dev/sdd1: LABEL_FATBOOT="EFI" LABEL="EFI" UUID="67E3-17ED" TYPE="vfat" PARTLABEL="EFI System Partition" PARTUUID="c2a64dbc-5b9a-458e-b0a8-04d6f5fd8956"
/dev/sdd2: LABEL_FATBOOT="WD1" LABEL="WD1" UUID="D719-1706" TYPE="vfat" PARTUUID="2cced532-4870-43f1-8226-4f413e513f33"

fdisk -l shows

GPT PMBR size mismatch (4294967294 != 5860533167) will be corrected by write.
Disk /dev/sdc: 2.7 TiB, 3000592982016 bytes, 5860533168 sectors
Disk model: EFRX-68AX9N0    
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: BFC5ECE6-8901-4C6C-A2BA-C14DA6AD5890

Device      Start        End    Sectors  Size Type
/dev/sdc1      40     409639     409600  200M EFI System
/dev/sdc2  411648 5860532223 5860120576  2.7T Microsoft basic data


Disk /dev/sdd: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: 01FALS-40Y6A0   
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 9B3E10E4-6E9B-4CE0-A7EF-691A6EA14CC5

Device      Start        End    Sectors   Size Type
/dev/sdd1      40     409639     409600   200M EFI System
/dev/sdd2  411648 1953523711 1953112064 931.3G Microsoft basic data

Is this a special thing related to the USB hub? Or is it normal that a FAT partition will also create an EFI System device?


EDIT: Funny, since I just wanted to and did nothing else then to format the disks:

 $ sudo mount /dev/sdc2 /mnt
 $ ll /mnt
total 132
drwxr-xr-x  4 pi   pi   32768 Jan  1  1970 ./
drwxr-xr-x 21 root root  4096 Jul 10 02:41 ../
-rw-r--r--  1 pi   pi    4096 Oct 12  2019 ._.com.apple.timemachine.donotpresent
-rw-r--r--  1 pi   pi       0 Oct 12  2019 .com.apple.timemachine.donotpresent
drwxr-xr-x  2 pi   pi   32768 Oct 12  2019 .fseventsd/
drwxr-xr-x  4 pi   pi   32768 Oct 12  2019 .Spotlight-V100/
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  • If i comprehend (that SCREEN?) then this is very normal. I was also surprised when I saw all the apple files after copying from a USB. Hidden files also. That is user friendly. Cookies. – user373503 Oct 12 '19 at 16:13
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    It just looks like Mac defaults to adding ESP with some boot files when you create a gpt partitioned drive. As it says use fdisk & do a write to clean up mismatch. Both Windows & Linux tools start first partition at sector 2048. It looks like Mac uses sector 40 which at least is aligned as it is divisible by 8. – oldfred Oct 12 '19 at 16:24
  • I agree, everything OK. Only me I mix up sectors and bytes. – user373503 Oct 12 '19 at 17:24
  • @oldfred the files are on the second (FAT) partition. the first (EFI) is actually empty. @rastafile SCREEN was just the terminal telling me that I am in screen number 0 and could close the terminal without interrupting a process attached. I removed it from the question. – Ben Oct 12 '19 at 18:54
  • @Ben hey thanks a lot, too. – user373503 Oct 12 '19 at 18:58
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You have two GPT formatted "disks". Both have a 200 MB EFI system partition.

sdc has "PMBR size mismatch", meaning protective MBR.

In other words maybe a mess...but the way you tell: with external multi-disk from a different system.

ADDED: I also don't like Start=40. I have 2048. So I have the first MB (?) "out of harms way" (harm=some MBR sector write). But it does say "size mismatch" and "will be corrected".

see comments for how we found the answers. And bottom WHY? for the big Q. I leave it like this. Thanks!

"The formatting I did" -- No joke and less insult: are you sure what you and macos exactly did?


WHY does mac do that?

It is a very good idea to reserve a 200MB (or a bit more even) partition in case you want to make the disk EFI bootable later.

(initrd plus kernel can be 50 MB together as files)

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  • no, not sure. i did blkid on linux and saw just the PMBR disk as /dev/sdc (without number) and tried to format it with ext4 on the terminal. then it was also not visible any more. then i plugged it into my Mac and both came up for initialisation. because ext4 is not availlable on the Mac i formatted both as FAT with the disk utility GUI. So i think the GUI does a bad job. i might redo the whole thing to ext4 now anyway since i now can See both disks but i need to be sure how to do it. – Ben Oct 12 '19 at 14:39
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    MBR partitioning can not be used on the 2.7 TiB disk, it is too big. 2 TiB is the maximum size for a 512 byte / sector disk. This is why you get the Protective MBR size mismatch warning. @rastafile: The disk has 4 k physical sectors, but 512 byte logical sectors, which is what counts in these calculations. – Johan Myréen Oct 12 '19 at 15:23
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    @ben but this PMBR is an option you can choose or not. It's how GPT moves it's first sector away from the edge. But sure, once you have it, it might stay. But having a PMBR, even octuple size would not be the problem. Try man wipefs for erasing "signatures" – user373503 Oct 12 '19 at 15:32
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    @ben the PMBR is a "protective" MBR. This is added by the GPT partitioning tool, and is supposed to cover the whole disk. It is meant to be a safety measure against old disk partitioning tools which are not aware of GPT. The old tool sees that the disk contains a partition that spans the whole disk, thus the disk has no free space. Without the PMBR, an old fdisk program might fool the user to think the GPT formatted disk does not contain any partitions, i.e. it is empty. The mismatch occurs because you can't have a 2.7 TiB MBR partition. – Johan Myréen Oct 12 '19 at 15:48
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    hybrid and PMBR are not the same – user373503 Oct 12 '19 at 19:48

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