HERE'S AN INSTANT WORKAROUND FROM A 3rd WORLD COUNTRY WHICH IS ACTIVELY BEING SUNK.
=====> I've prevented contact on pin 4 (GND) with a paper strip, careful not to prevent contact on other pins, especially pins 5-6, deeper in the connector.
Of course I'm talking about the USB connector which comes from the HUB and gets plugged into one of the Raspberry PI4 USB3 port.
Hypothesis: The signal lines have their own circuit, so interrupting either + or - of the power circuit (pins 1 and 4) shouldn't impede comms.
It works, boot/reboot fine, speed hasn't changed and it's OK for USB3.
On the other side of the cable I have a generic cheap powered USB3 HUB (small and fast, 4 ports, each one has a switch 0-1, I can't see any brand name, only "model ASX407" on a label added by the importer, I've bought the last two the seller had in stock and still I've spent about half of what I would have spent to buy one Transcend TS-HUB3K from abroad).
I'VE USED THE THINNEST PAPER I HAD HANDY, it turned out to be an ad deposited into my mailbox, I've cut a strip about as wide as one contact of a Yubikey 5 NFC (USB B), 2-3 cm long, I've put 4-5 mm of it into the connector, covering pin 4, and I've bent the rest on the connector chassis itself.
IMPORTANT NOTE: I haven't inserted the strip to the bottom of the connector, only 4-5 mm, then I've plugged the connector into the Raspberry PI4, which keeps the paper strip from moving (despite the fact that I have a fan pointing to it).
I was expecting to have to do multiple trials but it worked at the first one, I can now boot/reboot without having to temporarily disconnect the HUB power source, SPEED IS NOT LESS THAN BEFORE.
(The local market does not offer any of the HUB models with which there's no problem at boot/reboot time, buying abroad is being made more and more problematic, no commerce => plenty of activities die, it's NWO-ish, it's OFF-TOPIC though.)
This PI4's firmware is up to date.
Before the last update, this HUB was NOT giving I/O errors, but this WD Elements 4TB disk was, if connected directly to the PI4.
After the last update, on the contrary, I can also connect the disk directly to the PI4 without I/O errors (and I'm not getting low tension alerts any more, but still I prefer to power the disk from the HUB instead of having that current flow through the PI4).
When was the previous firmware update? A couple of years ago, I've let this thing sleep for some time (I had enough of wasting time looking for something which would not give these I/O errors on the picky USB3 port, with its makers apparently unwilling to publish any list of compatible devices and even closing the related GitHub thread which was full of hints by PI4 users [and complaints for the makers attitude]... recently I've told to myself that by now those problems could have been fixed by new kernel/firmware releases).