In the lawsuit, filed Friday in Suffolk County Superior Court in Boston, Carbonite said it bought more than US$3 million worth of Promise VTrak Raid products beginning in 2006. In several incidents starting in January 2007, the service provider suffered data loss because the Promise gear failed to support recovery from physical drive errors and array errors. The data losses caused "substantial damage" to Carbonite's business, the company alleged.

Promise responded that the suit was without merit. "Our investigation indicates that our products were neither implemented nor managed using industry best practices," the company said in a statement.

Based in Boston, Carbonite is a rival to larger services such as EMC's Mozy and Amazon's Simple Storage Service (S3) in the growing online backup market. The services let consumers and enterprises back up their data over the Internet for later retrieval if a hard drive or another component should fail. Carbonite targets its service toward home and small-business users.

In the complaint, Carbonite said it "lost the backups of over 7,500 customers in a number of separate incidents, causing serious damage to Carbonite's business and its reputation." Promise wasn't able to solve the problems despite a three-year limited warranty on its products, Carbonite said.

However, in a written statement following news reports on the case, Carbonite elaborated on the failures to say a smaller number of customers actually lost their own data. All customer backups involving the failed equipment were restored immediately and automatically, the company said.