How to fix CRC error when doing a SQL Server database backup? I'm in a pickle and any help would be greatly appreciated. I'm running our office's ERP Software (Acumatica) on a local server with Windows Server 2016 and SQL Server 2017.
Acumatica is currently running fine, we are not getting any errors whatsoever while using the app. We have 20+ employees constantly accessing Acumatica imputing data and so for. I have not receive any complains.
The problem that I'm having is that I'm getting an error when trying to backup the database using SQL Server Management Studio. When the backup process reaches 90% I get a SQL Server database cyclic redundancy check (CRC) error, and the backup fails.
I tried manually copying the .MDF and .LDF files to an external hard drive but the .MDF file fails just at the very end (99% with 120MB left to copy) with an error

Cannot read from source

I was able to copy the .LDF file. The .MDF file is around 140 GB in size and the .LDF file 180 GB.
What can I do to fix this and be able to do a full backup?.. The latest successful backup that I have is from 3 weeks ago. I really need to be able to backup the data before this gets any worse.
I have been thinking about running chkdsk with the repair command but I'm afraid it will make it worse and loose the database and/or data.
Any advice would be very highly appreciated. 
Regards.
Carlos
I'm having is that I'm getting an error when trying to backup the database

That's potentially bad.

I have not done a Transaction Log Backup ... I'm in Full Recovery Model

That's actually good. Then backup the log.  Somewhere safe.

In my test server I restored the backup that I have from 3 weeks ago

Almost home.  On your test server restore that old full with NORECOVERY, then restore the log backup.  If that works, your data is safe.  And even if your database is corrupt you can restore from the old full, the big log backup, and a Tail Log backup from the production server.