Comment 4 for bug 1875148

I have managed to work around this bug by doing the following:

1) Using James Chambers unofficial 64-bit build
Moving to 64 bit gets things moving, but after a several minutes the host controller would eventually hangup on me. I can't help but wonder if some 32bit ints are being blown somewhere because of the big reported sizes

2) Add the quirk 'usb-storage.quirks=[yourdrivesVID]:[yourdrivesPID]:u' to cmdline.txt
This limits DMA scatter-gather operations (new in Pi4) and seemes to help, but I've not been able to conclusively establish how

3) Connect the Drobo to the USB2 port rather than USB3
This (after the previous two steps) the finally kept the device mounted and working - kinda...

4) On another computer (I used a Mac with extFS added) repartition the drive to have a 16Tb partition (rather than the normal ~70Tb)
I found that if the OS tried to address data above some big value (never did manage to tie it down), perhaps while partitioning(!), formatting or fsck-ing, it would hang-up. In my case I happen to have 15Tb currently loaded in my Drobo and so I tried a 16Tb partition, and that worked.

5) In cmdline.txt change 'fsck.mode=auto' to 'fsck.mode=force'
I have found that the drive is always perceived as dirty and potentially corrupt (even though it isn't) on mount, so by forcing a fsck I ensure that the drive will be usable on boot (although this does mean that a reboot will make some functions unavailable for several hours.

6) Make sure that the mount options include 'nofail' and a good device-timeout, I use 'x-systemd.device-timeout=15'
The nofail ensures boot can proceed even though the fsck will take hours to complete (I have set the services that need the drive to wait for the mount), and the timeout caters for a corner case.
If it helps, my mount options in fstab are: 'rw,suid,dev,exec,auto,nouser,async,relatime,nofail,x-systemd.device-timeout=15'

And there it is. A big Drobo workaround, stuck on USB2 speeds, constrained to fraction of potential capacity, and that takes several hours to become usable... but I've got a NAS that works.

Obviously, this is far from ideal. I would love this to get fixed. Hopefully this has given some hints as to where to look, and helped others in the meantime.