HP50g - RESTORE - Help needed

[brrm]

Hi all, I need some help with restoring a backup on the 50g.

I created a back-up file on the SD card, which is port 3

The file name is BU_20210211.153

In RPN when I enter the file name and press ENTER I get an error saying 'Too Few Arguments'
and the stack shows 1: 3:BU

In Algebraic mode when I type RESTORE(:3:BU_20210211.153) and press ENTER I get an error saying: Invalid Syntax and the part of the file name following the underscore is highlighted.

So apparently something is wrong with the underscore however I don't understand what.

Help is appreciated.