Silent about the data loss for more than two years, the company published its version of events on Saturday following a report in German news magazine Der Spiegel that the data were being offered for sale on the Internet.
Data on the disk included customers' name, date of birth, address and mobile phone number, and in some cases the customers' e-mail addresses. No banking details were lost, the company said.
When the loss of the disk was discovered, the company reported the loss to the state prosecutor, and began monitoring Internet forums and sites where such stolen information is offered for sale, it said.
T-Mobile found no evidence in the months following the loss that the missing data was on the market, it said.
That changed on Saturday, however, with Der Spiegel's revelation that the data is now for sale on the Internet.