Offers of free (or cheap) storage are tempting, but don't add an account just because you can. Each cloud storage account you use should serve a unique and useful purpose. I cancelled my accounts with several providers because they all duplicated capabilities I already got elsewhere. On the other hand, I keep Google Drive and iCloud Drive, despite their similarities, because each one offers features the other doesn't: namely, integration with the provider's proprietary software.
Don't confuse cloud storage and cloud backups
Cloud backup services such as CrashPlan copy files to distant servers, and let you retrieve those files from another computer or an iOS app. That sounds a lot like cloud storage. On the other hand, Dropbox stores deleted files and old versions for 30 days, or up to a year if you pay extra for Extended Version History. That sounds a lot like cloud backup.
But services that specialize in storage are generally better at keeping your files in sync across devices, while services that specialize in backup are generally better at long-term retention and data restoration (and often have superior encryption, too). Each service meets a different need, so I don't consider cloud storage and cloud backup of a given folder to be redundant. I use both.
Let each service stand alone
Suppose you use iCloud Drive because that's what Keynote works best with, and Google Drive because that's what Google Docs works best with. Fair enough — let each service hold its own documents. If the two sets of files sync independently with your Mac (and in most cases they will), that's even better. But trying to sync all your documents between cloud services is usually a waste of effort (and perhaps, depending on how you do it, a waste of money). That brings me to the next point.
Use aggregators only as needed
Providers such as cloudHQ, Otixo, and ZeroPC let you aggregate cloud storage services — that is, after you connect all your accounts, you can see your documents from every provider in a single view in the Web or an iOS app, drag files from one service to another to copy or move them, and in some cases even sync files between cloud services.
It's a neat trick, and can be a big help if you have files scattered across many services. But although basic plans are free, you may have to pay as much as your cloud storage itself costs for full-featured aggregation. Besides, if you're following the previous tip, you should seldom need to move files from one service to another — and even when you do, you can use your Mac as a conduit and avoid paying for a cloud-to-cloud transfer service.
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