| Data Loss & Incomplete Backup: Hidden Dangers of Dropbox, OneDrive, iCloud, & Google Drive
The Problem
Cloud-sync services like Dropbox, OneDrive, iCloud, Google Drive, and others (which synchronize certain folders between your computer and their online storage) usually work as intended. However, there are some subtle aspects of these services that may end up deleting the contents of many of the documents on your computer, with no disclosure in advance nor any warning in the moment. When things are working correctly All of these services work in a similar manner:
- You set up a cloud account (online, choosing a username and a password) with the given company. That account may be free (with a basic amount of online storage), or it may have a monthly or annual cost (with larger storage). This might have been done years ago.
- You set up the service’s synchronization software on your computer, which will monitor one or more specific folders on your computer.
- Then, any changes that you make in those particular folders on your computer to any of the files and folders (and their subfolders) get uploaded to that cloud storage right away, including creating, editing, renaming, moving, and deleting.
You might think of this as a “partial backup,” protecting some of the data on your computer by storing copies of those particular folders in that cloud account. Issue #1: Fully-present files vs. Placeholders The software provided by each of these services has a special setting which is usually turned on by default: Over time, the synchronization algorithm notices which of the documents (in the synchronization folders) you are actively using (opening, editing, etc.) vs. which ones you have not looked at recently. What happens next is the core issue: After a period of time (which is not documented), the software quietly replaces each inactive document on your computer with a placeholder. This special file has the same name and icon as the original but no content. As a result, this process frees up the disk space that the file originally consumed without notifying you that this “dehydration” has happened. The full content of the copy stored in the cloud is preserved. This process may also get triggered sooner if your computer runs low on disk space. Issue #2: Your computer is now dependent on the cloud for the content of those documents If you later try to open a placeholder document (and everything is working properly), the software immediately steps in and downloads its contents, “re-hydrating” it to be fully-present again. The larger your collection of documents, the more likely that many of them will become inactive. Depending on the amount of storage they consume, this “dehydration” mechanism may free up a noticeable amount of disk space while simultaneously making your computer dependent on that cloud account to access these inactive documents. Issue #3: These placeholders will immediately undermine the integrity of your regular backup If (as I recommend) you use regular backup software to back up all of your data (not just your synchronization folders) to an external drive or an online backup service, that backup software cannot access the original documents represented by the placeholders, and it may not report any errors, since the placeholders may act like real files or get quietly skipped. This means that if you later had to restore from one of those compromised (incomplete) backups, only then would you discover that your backup does not contain the contents of those “dehydrated” documents. Issue #4: Your computer may get into an “overflow” (“oversubscribed”) situation If you’re very actively creating (or downloading) new files, when they arrive they will naturally use some of the free space on your computer. If you store those new files in your synchronization folders, the ones that become inactive will eventually get turned into placeholders, freeing up the space they originally consumed. If this cycle continues, over time the placeholders in your synchronization folders may collectively represent so much disk space that there is no longer enough free space to turn them all back into fully-present files. This problem can be fixed, and (unless you are willing to discard enough of those placeholders) you’ll probably need to start by getting additional internal or external disk storage. After that it gets complicated. Issue #5: Your computer may get disconnected from that online account That “re-hydration” mechanism (when you try to open a placeholder) may stop working for a variety of reasons, including:
- You computer may (temporarily) not be connected to the internet.
- Your cloud sync account password may have changed.
- Your sync software may get signed out or disconnected from your account, or connected to a different account.
- Your sync software may get turned off or stop working or get uninstalled.
- You might have moved to a new computer and the sync software wasn’t set up correctly (or at all).
If re-hydration isn’t working, trying to open a dehydrated document (i.e., a placeholder) will either get an error or display no content. Also, if you’re paying for extra online storage but your cloud sync account subscription expires for lack of payment, re-hydration will probably still work but synchronization will stop. What can you do about these placeholder and overflow problems? Ideally, as soon as you set up any cloud synchronization software, you would immediately change its setting to only have fully-present files and never make placeholders. That’s the best way to prevent these problems from occurring. However, if you’re already using a cloud synchronization service, before changing anything it’s best to first assess whether your computer is in an overflow state. The basic steps are:
- Estimate the amount of collective disk space that all of the placeholders represent, using Properties on Windows (Get Info on Macintosh) for each of the top-level synchronization folders. Look for the size of the data represented. If it’s noticeably larger than the data size consumed, then you probably have many placeholders.
- Compare that “represented” size to the current free space on your computer’s internal hard drive. If the free space is smaller, then you’ve got an overflow situation.
Note that you may not be permitted to change that setting (to prevent or re-hydrate all placeholders) if your computer is managed by an employer or some other large organization. Quick summary of sync services As of this writing, here’s a brief overview of the sync services I’ve mentioned:
- iCloud Drive (Apple): 5GB free, paid starts at 50GB; turn “Optimize Mac Storage” on
- OneDrive (Microsoft): 5GB free, paid starts at 100GB; click “Download all files” (not “Free up disk space”)
- Dropbox: 2GB free, paid starts at 2,000GB, click “Make available offline” (not “Online-only files”)
- Google Drive for desktop: 15GB free, paid starts at 100GB (shared with Gmail and Photos); click “Mirror files” (not “Stream files”)
Note that your computer might have placeholders due to multiple sync services. For example, Dropbox might be active now and OneDrive might have been active in the past, each having replaced some of the fully-present documents with placeholders in their respective folders. Where to go from here As always, if any of my advice seems too difficult to follow, I recommend that you find someone you know and trust who can help you check whether this mechanism might be active on your computer, and then correct it appropriately if it is. In the searches below, replace “X” with the name of your cloud synchronization service, e.g., Dropbox, OneDrive, etc.
- google: X placeholder
- google: X offline access
- google: X oversubscribed
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