Mozy has announced the days of its unlimited backup service are limited.  What the online backup provider used to sell “for the price of a hamburger” ($4.95 a month) is about to match Angus Beef prices.

Why the price increase?  Because people actually used the service to backup their hard drives.

“There has been a change in consumer behavior,” Russ Stockdale, Mozy’s vice president of product management, told CNET News. “What we have seen since we launched an unlimited service five years ago is there has been an explosion in digital content, specifically digital photos and video.”

The new plans, to be unveiled today:

• Mozy Basic: $5.99/mo for up to 50GB of data.  Each additional 20GB increment will cost $2 more per month;
• Mozy Multi-Machine: $9.99/mo for up to 125GB of data from up to three computers.  Each additional 20GB increment is also $2 more per month.

Earlier days.

CNET columnist Stephen Shankland is taking personal responsibility for the company’s price increase, noting he is among the service’s top 0.3 percent of users, backing up nearly 600GB of his digital media files — exactly the kind of customer Mozy wants to charge more.  His multi-year discount means the $3.40 a month he used to pay will now rise to $60.

Mozy’s competition:

• Google Docs costs $1,400 a year for 400GB;
• Google’s Picasa Web Albums costs $100 per year for 400GB.
• Jungle Disk, which provides a front end to storage using Amazon’s S3 service, charges a flat rate of $3 per month plus 15 cents per gigabyte per month.
• Carbonite, perhaps Mozy’s best-known competitor, throttles down bandwidth for big-data users.
• Dropbox charges $20 per month for 100GB.

One service the usage cap-happy broadband industry will be certain to threaten is online file backup.  Consumers who don’t know any better can easily configure software to back up entire hard drives to a remote hard drive, blowing through an online usage allowance in a matter of days.  Even usage allowances as large as Comcast’s 250GB per month are no match for today’s super-sized hard drives.

So it comes with a bit of irony that Comcast has quietly launched its new Secure Backup & Share service, “powered by” Mozy.

Every Comcast broadband customer will soon be pelted with promotions for the new free add-on, which will initially provide 2GB of storage space.  The free version is enough to backup small collections of music, photos, and documents, and probably won’t hurt your allowance too much.  But Mozy gets to up-sell customers to their much-larger capacity plans right from the home page.  A year’s worth of 50GB of storage costs $50.  Get 200GB of storage space for $100 a year.

Exceeding 250GB of usage per month, with or without the service, will potentially get you a warning letter and then an account suspension.

Bonus points to you if you can find the 250GB usage limit disclosed on the home page for the service.

For providers who try for far lower usage allowances, or charge up to $2 per gigabyte after exceeding them, an online file backup service could make your provider’s day once they send you the bill.