Like SkyDrive, Google Drive won't delete your files after the promotion ends. You'll still be able to access, share, and download those files, according to Google's support page, but you won't be able to add any new files until you either clear space or purchase a plan that covers the amount of space you're using.
Gmail users need to treat Google's promotional offerings with caution. Going over your limit means that you won't be able to send or receive email "after a period of time," according to Google's support page. Google will provide warnings in advance of expiration of the free storage period, but a company spokesperson would not specify the length of the grace period users will have before their email gets cut off. Likewise, syncing between your Drive account and the Google Drive folder on your local hard drive "stops completely" when your plan expires.
If you decide to pay for Google Drive storage beyond the free period, you should know that plans start at $5 per month for 100GB, and you'll have to pay $50 per month to maintain the same 1TB of storage that comes with the Chromebook Pixel.
Although Dropbox isn't in the hardware business, the company has partnered with device makers such as HTC and Samsung to offer free cloud storage with their phones. These promotions may last a couple of years, and some offer as much as 48GB of extra storage.
Dropbox's support page says that it won't delete files after you've reverted to a regular free account. You can still access those files from your computers, tablets, and phones, and from the Dropbox website. But if you try to add new files to the Dropbox folder on your PC, they won't sync, meaning that you won't be able to access them from other machines. Similarly, you won't be able to upload new files via Dropbox's apps or website.
Dropbox isn't tied to vital tools like your email or Office applications, which should help limit the potential fallout from an expired free plan. On the other hand, its paid plans are more expensive than those for Google Drive and SkyDrive. Pricing starts at $10 per month for 100GB of storage.
Just plan ahead!
Having lots of extra cloud storage can be convenient for shuttling large files across your devices. But don't fall into the habit of considering these promotions truly free.
In all likelihood, your phone or tablet won't have enough local storage to contain all the files you've been putting in the cloud. Consequently, when the promotion ends, you'll need a PC with ample storage or an external drive for backups, or you'll have to start paying recurring fees for cloud storage—and you'll have to make a decision fast if you don't want to disrupt your work. You know what they say about free lunches.
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