Why can't I copy and paste files from Mac Studio internal drive to an external HDD with read-only staff permissions?

Drag and Drop really is the preferred method for moving files. Copying a file reference to the clipboard is mainly intended for manipulating NAMES of items, not the items themselves.

I know Drag-and-Drop is hard with a trackpad, but that is likely because you did not PRE-Open a SECOND Finder Destination window into which to DROP the items. I posted instructions above and now have made an example graphic to go with it:

Open the Source window in Finder.

Choose 'NEW Finder Window' off the Finder's File menu.

'Navigate'-- that is adjust the destination showing in that SECOND window.

Select an item or many items in the SOURCE window.

Drag and Drop to the Destination window, in ONE simple movement and no fiddling or opening things while you are holding files on the pointer.

if you are concerned that this is a Permissions problem, drag and drop one file and see if that goes according to plan.

the easiest way to do this is to open the Source directory, then choose Finder > New Finder Window, and navigate the second window to the Destination. With source and destination in sight, drag and drop one file.

¿Does it go?

-- yes, permissions is not the issue

-- no, permissions may be the issue

No, that's not the way it works.

If you want to modify what you said, your additions and edits are treated as an "audit trail" and the lead-in statements and questions and answers are preserved.

Interestingly, this also preserves the thought process that lead to your initial statement, and show the resolution or difference as the discussion unfolds and/or different information becomes available. This often changes what you Would have written, if perfect information were available at the start. This can actually be MORE valuable for Readers facing similar issues later.

You do have a brief window to fix glaring errors, which may be on the order of ten minutes. After that, "the ink dries" and you can only edit by adding an 'erratum slip' by posting additional postings.

I'm not the OP but I too hate dragging and dropping on Mac because it's so fiddly. If the folder has a mix of folders and files then it's a 50/50 chance that whatever's being dropped will drop into a sub folder rather than the folder. You have to line everything up beforehand and make sure the destination folder is scrolled to just where you want it because scrolling while dragging won't work when you need it to or it will scroll so quickly so as to be useless. The time for folders to open automatically is set randomly and maliciously to be too quick on folders you don't want and too slow on folders you do want. Once the wrong folder is open there's no way to go back to where you want. On more than one occasion I've turned off my Mac with the power button while holding on to a bunch of files to avoid having to drop them in the wrong place. It's awful and I have total sympathy with the OP.

Practice makes perfect.

I've been dragging and dropping for over 25 years without having any issues.

If the wrong folder tries to accept the drop, all you have to do is backup and/or move to the right one.

What I really hate are Magic Mice are Trackpads.

<< while holding on to a bunch of files to avoid having to drop them in the wrong place >>

'The right place' when things are going badly is onto the TITLE area of an opened widow, which essentially says "forgetaboutit" and does not move anything.

I'm just curious about the specific difficulty you encounter with the Drag & Drop. I know with the Apple Trackpads with Haptic Feedback, that I cannot do anything correctly as long as the Haptic Feedback setting is enabled. Is this perhaps part of the issue you may be having with the Drag & Drop? Try turning Haptic Feedback off within the Trackpad System Settings to see if this makes any difference for you.

Another thing I do is to disable the Tap to Click feature since there is no adjust available. I get into so much trouble with Tap to Click enabled since it is usually much to sensitive for me.

If the Drag & Drop issue you have is with trying to accurately drop the items at the correct location, then this can indeed be tricky at times especially if you hover the pointer over a folder too long which will open it. The trick is to hover over the white space area between the icons in Icon View, or far enough to the right if using List View (there is a sweet spot where you can hover in List View).

I'm very dangerous with a Trackpad, but I do manage for the most part by customizing those Trackpad settings & learning to hover over the white area sweet spots so folders do not automatically open when hovering too long. You can hover over the back arrow in the Finder to close the automatically opened folder to return to the previous folder as well.

Maybe these things can help you as well.

I don't think the drives are the problem with the Copy & Paste action.