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.

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https://bugs.launchpad.net/bugs/1875148

Title:
  USB ports on Raspberry Pi are deactivated when connecting a Drobo unit

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