The Omaha-based company mailed the notices to its clients last week, spokeswoman Donna Kush said.
The company realized the tape was missing in February, when the package it was in was damaged during shipping between vendors, Kush said. Of the four backup tapes in the package, three were found, but the fourth is still missing, she said.
Information on the tape was for people nationwide who may have been Ameritrade customers from 2000-2003, she said. The data was different for each client and may have included their Social Security numbers, among other information, Kush said.
The tapes were not marked and unless special equipment was used, the compressed data could not be extracted.
Kush said she has not heard of any misuse of the information.
"We are very confident that no harm has been done to any clients, to their accounts, to their information," Kush said.
Kush said Ameritrade did not lose the information, but rather, a third party vendor did.
The news comes as several companies have experienced their own database violations, and some thefts.
Database giant LexisNexis on Monday announced it had started alerting about 280,000 people that who were using stolen passwords and IDs.
On Monday, Columbus, Ohio-based DSW Shoe Warehouse said that on about 1.4 million customers. The company said it has contact information for about half of those people and started sending letters notifying them of the thefts, which happened at 108 stores in 25 states between November and February.
Data broker ChoicePoint Inc., based in suburban Atlanta, said in February that who opened up dozens of accounts and went undetected for more than a year.
By Emily Fredrix