In our courses we often see delegates trying to copy cells to the left and making an incorrect assumption on which part of the cell to click when using the mouse. So in order to copy a cell to the left in Excel, you need to start in the same place as when you copy right.
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YouTube Copy cells to the left
Copy a cell to the right in Excel
So to copy to the right you can hover your mouse at the bottom RIGHT corner and you will see that the cursor changes to a simple cross (see below). If you click and drag this to the right it will copy the cells.

It is VERY important that you only click and drag when you see this cursor (simple cross). If you accidently move slightly up and the cursor changes to a cross with arrows (as below), you will then be moving and not copying which can cause major problems for your spreadsheet.

Copy a cell to the left in Excel
In our courses we see people trying to get this cursor by hovering over the bottom LEFT of the cell. This is incorrect. Even if you want to copy left, you need to hover over the bottom RIGHT and then drag to the left.

Again, be careful of clicking when you get the pointed cross as apposed to the simple cross.
Related
Copy from left to right without going all the way to end of the sheet