Does cloud storage affect your files in some way?

Question

For example, I have created ISO images in Imgburn that I tested and opened quickly with 7-Zip, but I have uploaded them to the Cloud on Google Drive, for example, and when I downloaded them again, they would test quickly as usual, BUT 7-Zip would take a LOT longer to open them. I don't know if Google Drive did something to the ISO or not. Should there be any difference in the files at all or not?

Answer 1

What you are likely seeing is that your initial testing was working from the operating system disk cache while later accesses were needing to scan the file from disk. As Biswa mentions in a comment the files should be identical and it can be verified by running a checksum against both versions.

If you have a good amount of memory and tested the files just after creation then there is a good chance that the entire file, or a significant portion of it, would be in the disk cache and reading the file to test it would be extremely fast.

If you tested the file immediately after download, and the system did not have any significant disk access during download, then again it should be in the disk cache and would be fast to open or test.

If the download happened over several days of startup/shutdown cycles, or the system was doing something else with the disk during download, then the file would be being accessed from disk rather than from RAM and would be significantly slower to access. After the first time you access it or read the entire file then it should be "fast" again.

Answer 2

Does cloud storage affect your files in some way?

Potentially, yes. The typical change is additional compression, especially for images and video. This doesn't mean all (or any) of your files are definitely going to be affected but the possibility it certainly there.

I don't know if Google Drive did something to the ISO or not. Should there be any difference in the files[?]

Ideally there should be no difference. What you describe (opening slowly with 7-zip) could be attributable to something Google did, but it could also be something as simple as a fragmented file or some other issue resulting from the transfer (either during initial upload or subsequent download).

As mentioned in the comments, you should be creating checksums (which can be made with a variety of programs) to help verify data integrity. Any checksums should be made before any initial upload (i.e. when you know you still have a proper file).