HTCSense.com users can salvage their files by logging in and downloading a ZIP file before April 30, when all stored content will be deleted.
The firm explained that the online service is undergoing a "renovation" and pledged to notify customers as soon as its successor is ready to go live.
"Until the new services are ready, features previously available on HTCSense.com will be shutting down," said HTC. "You'll be among the first to know when the new and improved services are ready."
HTC is about to launch its One series of smartphones following their unveiling at the Mobile World Congress earlier this year. The handsets come with a two-year subscription to Dropbox's premium service.
The One X, One S, and One V will begin shipping in April across more than 140 mobile operators and distributors globally.
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Watch an introductory video for HTC's One line below: