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Sorry that you're getting an error that Photoshop 2020 could not save the file because of a disk error.
Are you getting the error while saving a specific file to a specific location? Does the issue occurs with all the files? Also, which version of macOS are you running?
Please check that you have the latest version of Photoshop 2020(v21.2) installed. See this link for more details: https://helpx.adobe.com/photoshop/kb/uptodate.html
To troubleshoot the error you're getting while saving, please checkout the steps mentioned in this article https://helpx.adobe.com/photoshop/using/scratch-disks-preferences.html#Troubleshootscratchdisksarefu... and let us know how it goes.
my file is open inside external hardisk
Try to move the file directly to your computer and check if it opens correctly or the error appears again.