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?
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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.
Mokubai
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I just uploaded and then downloaded variously-sized ISOs from three different cloud storage websites on the same day, just a couple of hours apart, and the downloaded ISOs are taking a lot longer to individually open in 7-Zip. Will it speed up after a while? – Foebane Dec 10 '17 at 17:02
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They will be faster the second time you access the file in quick succession. If you access the first, close it, then open it again then it will be quick. Accessing one file, then another and then another again will likely push the first out of the cache and make it slow again. It's just how these things work – Mokubai Dec 10 '17 at 17:21
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It hasn't made any difference, it takes longer to open the downloads (busy circle) and the delay seems to be longer relative to the size of the file. I even tried extracting the contents of the downloaded ISOs to a temporary folder to see if that would help, but no. I even restarted Windows and tried to open the downloads first. HOWEVER, when I opened the files one by one within the 7-Zip file manager, access was instant then. My worry is that somehow, the downloads are somehow accessing the cloud storage sites for permission before they'll open, or something. – Foebane Dec 10 '17 at 20:38
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"when I opened the files one by one within the 7-Zip file manager, access was instant then" that is what I mean, you need to do this step in order to "load" them into the disk cache for instant access. It is highly unlikely that they are looking for any permission to open them, it is simply that they are slow to access from disk. – Mokubai Dec 10 '17 at 20:48
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Sorry, let me explain: the three files are visible in File Explorer in Windows 10, and for each one I click on, they all take a few seconds to load (busy circle), 7-Zip file manager appears, and closing is instant. But if I move from one ISO to another within 7-Zip file manager, then it's instant. But each time I reopen them from File Explorer, it's the same delay. It's annoying, but the CRCs check out fine and they test fine, so I don't know why the cache isn't updating. Maybe it takes time? It could be because I'm opening the downloads in a different folder than the original ISOs? – Foebane Dec 10 '17 at 20:55
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It sounds like your problem is the loading of 7-zip and is completely unrelated to where the files are stored. – Mokubai Dec 10 '17 at 20:57
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Well, the original ISOs open fast enough. The uploaded ISOs are backups, and if I ever need to use them, then I could just simply extract each one and create a new ISO from the contents, and I'd be alright again. – Foebane Dec 10 '17 at 23:51
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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).
Anaksunaman
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2I used 7-zip to create a SHA256 checksum from the original file, and compared it to the version I downloaded (that I uploaded before) and the checksums were identical. So I think it could be down to the OS disk cache. – Foebane Dec 09 '17 at 10:51