hard drive – Ubuntu in external HDD not recognized as boot option when plugged in USB 3.0 port

I found this similar question yet unsolved.
I’ve installed Kubuntu 20.04 on an external HDD, with Grub boot menu, configured for EFI (I’m not sure about this one). The exHDD is a USB3.0 HDD case/box. When connecting to USB 2.0 port, it shows up in the boot option list but doesn’t when connecting to USB 3.0 port.
I’ve tried on several PC:

  • My laptop, Lenovo G510, is described above. However, there are 1 or 2 times it does bootable with USB 3.0 but never seen again.
  • PC1, MSI motherboard, boot fine with both USB 2.0 and 3.0.
  • PC2, Aorus motherboard, only boot with USB 2.0.
  • My school’s PCs, boot fine with both USB 2.0 and 3.0/3.1.

Boot with USB 2.0 noticeably reduces performance. The main purpose of that exHDD is being my portable Linux system.
What causes this and can this issue be fixed or I must live with it?