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I had the same "permission denied" problem and tried all the earlier suggestions with no improvement. I was using the built-in MicroCard port of a UGreen brand thunderbolt port expander (MBP2019-Intel, plus an older Intel-based machine plugging it into a USB port using a converter). Also tried several Linux VMs, in Parallels and VirtualBox, on the same machines. The thunderbolt port expander does correctly read/write ordinary files on MicroSD cards.

Then I tried plugging an older UGreen USB-based card reader into the UGreen thunderbolt gizmo. That worked on the MBP; it's blazingly slow.

I've previously been disappointed with other UGreen devices, but all the low-end options look like they use the same boards with different casings for branding. They all get crappy ratings. (Hint: don't use the charging port if you can avoid it.) I'm not convinced the more expensive ones would be any better, but maybe I'm too pessimistic.

Answer: OP steps are correct. If you follow all the earlier caveats, the problem is (likely) due to the hardware you are using.

I had the same "permission denied" problem and tried all the earlier suggestions with no improvement. I was using the built-in MicroCard port of a UGreen brand thunderbolt port expander (MBP2019-Intel, plus an older Intel-based machine plugging it into a USB port using a converter). Also tried several Linux VMs, in Parallels and VirtualBox, on the same machines. The thunderbolt port expander does correctly read/write ordinary files on MicroSD cards.

Then I tried plugging an older UGreen USB-based card reader into the UGreen thunderbolt gizmo. That worked on the MBP; it's blazingly slow.

I've previously been disappointed with other UGreen devices, but all the low-end options look like they use the same boards with different casings for branding. They all get crappy ratings. (Hint: don't use the charging port if you can avoid it.) I'm not convinced the more expensive ones would be any better, but maybe I'm too pessimistic.

I had the same "permission denied" problem (MBP2019-Intel, plus an older Intel-based machine plugging it into a USB port using a converter) and tried all the earlier suggestions with no improvement. I was using the built-in MicroCard port of a UGreen brand thunderbolt port expander. (Also tried several Linux VMs, in Parallels and VirtualBox, on the same machines.) The thunderbolt port expander does correctly read/write ordinary files on MicroSD cards.

Then I tried plugging an older UGreen USB-based card reader into the UGreen thunderbolt gizmo. That worked on the MBP; it's blazingly slow.

I've previously been disappointed with other UGreen devices, but all the low-end options look like they use the same boards with different casings for branding. They all get crappy ratings. (Hint: don't use the charging port if you can avoid it.) I'm not convinced the more expensive ones would be any better, but maybe I'm too pessimistic.

I had the same "permission denied" problem and tried all the earlier suggestions with no improvement. I was using the built-in MicroCard port of a UGreen brand thunderbolt port expander  (MBP2019-Intel, plus an older Intel-based machine plugging it into a USB port using a converter). Also tried several Linux VMs, in Parallels and VirtualBox, on the same machines. The thunderbolt port expander does correctly read/write ordinary files on MicroSD cards.

Then I tried plugging an older UGreen USB-based card reader into the UGreen thunderbolt gizmo. That worked on the MBP; it's blazingly slow.

I've previously been disappointed with other UGreen devices, but all the low-end options look like they use the same boards with different casings for branding. They all get crappy ratings. (Hint: don't use the charging port if you can avoid it.) I'm not convinced the more expensive ones would be any better, but maybe I'm too pessimistic.

I had the same "permission denied" problem (MBP2019-Intel, plus an older Intel-based machine plugging it into a USB port using a converter) and tried all the suggestions above with no improvement. I was using the built-in MicroCard port of a UGreen brand thunderbolt port expander. (Also tried several Linux VMs, in Parallels and VirtualBox, on the same machines.) The thunderbolt port expander does correctly read/write ordinary files on MicroSD cards.

Then I tried plugging an older UGreen USB-based card reader into the UGreen thunderbolt gizmo. That worked on the MBP; it's blazingly slow.

I've previously been disappointed with other UGreen devices, but all the low-end options look like they use the same boards with different casings for branding. They all get crappy ratings. (Hint: don't use the charging port if you can avoid it.) I'm not convinced the more expensive ones would be any better, but maybe I'm too pessimistic.

I had the same "permission denied" problem (MBP2019-Intel, plus an older Intel-based machine plugging it into a USB port using a converter) and tried all the earlier suggestions with no improvement. I was using the built-in MicroCard port of a UGreen brand thunderbolt port expander. (Also tried several Linux VMs, in Parallels and VirtualBox, on the same machines.) The thunderbolt port expander does correctly read/write ordinary files on MicroSD cards.

Then I tried plugging an older UGreen USB-based card reader into the UGreen thunderbolt gizmo. That worked on the MBP; it's blazingly slow.

I've previously been disappointed with other UGreen devices, but all the low-end options look like they use the same boards with different casings for branding. They all get crappy ratings. (Hint: don't use the charging port if you can avoid it.) I'm not convinced the more expensive ones would be any better, but maybe I'm too pessimistic.