Control Data Corp., the Minneapolis-based computer firm, reported a $54.5 million loss for the third quarter yesterday, largely the result of a one-time $70.3 million after-tax charge related to the decision to halt production of large disk memories compatible with mainframe disk drives built by International Business Machines Corp.

Control Data has tried unsuccessfully since 1981 to produce a disk data-storage unit compatible with IBM's Model 3380 drive. But after Control Data shipped the first 500 of its sophisticated drives early this year, it halted production due to product defects.

The company's difficulties were compounded when competition hurt profit margins on the drives after IBM cut prices on its drives in September.

"All the way across a number of Control Data's businesses, anything that can go wrong is going wrong," said Thomas J. Crotty, an analyst with the Gartner Group. "My expectation is that the company will survive in the future, but as a much smaller company."

The $54.5 million third-quarter loss ($1.41 a share) on revenue of $1.2 billion announced yesterday compares with earnings of $40.7 million ($1.05) on revenue of $1.2 billion in the third quarter last year.

Excluding the one-time charge from the phase-out of the plug-compatible business, the company earned $15.8 million in the third quarter. The decline in third-quarter earnings was also because of weak performance in the computer services area, the company said, noting that financial services earnings in its commercial credit unit increased over the third quarter of 1983.

"This is the culmination of a lot of problems," one securities analyst said. "They have had terrible on-going technical problems which led them to be two years late in entering a market where IBM has gotten much more aggressive. The computer services business is also giving them terrible problems."

The company said it expects substantially depressed earnings for the year, compared with 1983's $161.7 million ($4.20) net income on revenue of $4.6 billion, but expects fourth-quarter profits to reflect a substantial increase over third-quarter operating earnings because of improvement in computer services and other sectors of the business.

For the first nine months of 1984, Control Data earned $600,000 (one cent) on revenue of $3.7 billion, versus $113 million in net income ($2.94) on revenue of $3.3 billion in the first nine months last year.

Control Data stock closed at 30 5/8, unchanged yesterday. The company had previously disclosed the problems to securities analysts, and the value of the stock has fallen during the last year from a high of 49.

Analysts had been counting on sales of the disk memories to add up to $200 million to Control Data's 1984 revenue. But that was before the company's decision to give up trying to make the large disk drives that are plug-compatible with the latest generation of IBM storage devices.