File → Open Data Folder and verify you put it in the correct location.
Thanks William. I normally access these locations by going to File/Open Data Folder so I'm quite sure the file is in the locations that I specified above.
What I am not sure of is whether these are the correct locations when one is using the strategy tester and the #property tester_file File directive.
Any help on this appreciated.
algodevtrader:
Hello,
I am having the well known problem of the strategy tester erasing files that I am reading in my EA. I have read the documentation
.....
Anyhow, much appreciate any assistance.
There is an older and similar thread here about this:

Open Files during backtest
- 2018.05.16
- www.mql5.com
Hello, when i place a file into folder files i can open this file when running or debug my expert...

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Hello,
I am having the well known problem of the strategy tester erasing files that I am reading in my EA. I have read the documentation
and the forums and taken all advice given so far. When running on a chart, my EA reads from this directory:
C:\Bunchofstuff\MetaQuotes\Terminal\INSTALLID\MQL5\Files\File.csv
When running in the strategy tester, my EA reads form this directory:
C:\Bunchofstuff\MetaQuotes\Tester\INSTALLID\Agent-127.0.0.1-3000\MQL5\Files\File.csv
This line is at the top of my EA code.
#property tester_file "file.csv"
The file still almost always gets erased from the Tester directory.
I say sometimes, because MT5 seems to sometimes run the strategy test without erasing the file. However, it quickly gets into a weird state
and starts erasing again. I can sometimes get it to work again by killing and restarting MT5. This behavior is not reliably reproducable so I am
either missing something or their is a strange bug in MT5 (I know, that's impossible! ;-)
Anyhow, much appreciate any assistance.