On Thu, 7 Nov 2019 13:16:29 +0000, Areligious Republican
 wrote:

> Perhaps there is a need for a THUNK processor to
> emulate the SD protocol on one side (to replace the
> SD in Pi) and to drive a HD on the other.
>
> Job for an Arduino or a DSPic33?

Much too complicated.

As Chris Elvidge says in message:
  Subject: Re: External disk with the pi 4, anyone
  Date: Thu, 7 Nov 2019 13:10:14 +0000
  Message-ID: 
, it is much easier to use the SD card just for /boot,
and put the rootfs on a harddisk

My setup is usually :

pi@f7p4:~ $ lsblk -o name,fstype,mountpoint,label,parttype,partlabel,partuuid

NAME        FSTYPE MOUNTPOINT LABEL   PARTTYPE PARTLABEL PARTUUID
sda
+-sda1      vfat                      0xc                32ebda7d-01
+-sda2      ext4   /          rootfs  0x83               32ebda7d-02
+-sda3      ext4   /extra             0x83               32ebda7d-03
mmcblk0
+-mmcblk0p1 vfat   /boot      boot    0xc                5e3da3da-01
+-mmcblk0p2 ext4              rootfs  0x83               5e3da3da-02

pi@f7p4:~ $ cat /boot/cmdline.txt # (all on one line!)
console=serial0,115200 console=tty1 root=PARTUUID=32ebda7d-02 rootfstype=ext4
 elevator=deadline fsck.repair=yes rootwait

pi@f7p4:~ $ cat /etc/fstab
# device              mntpnt          type    options        dump  passno
proc                  /proc           proc    defaults          0       0
# SD card
PARTUUID=5e3da3da-01  /boot           vfat    defaults          0       2
# SSD / HD
PARTUUID=32ebda7d-02  /               ext4    defaults,noatime  0       1
PARTUUID=32ebda7d-03  /extra          ext4    defaults,noatime  0       2
# a swapfile is not a swap partition, no line here
#   use  dphys-swapfile swap[on|off]  for that

Note: /extra is a partition for data / homedirs, that will
survive replacement of boot/root.
--
Regards,
Kees Nuyt

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