By late this morning, NHS Greater Glasgow and Clyde postponed 459 outpatient appointments, 14 inpatient procedures, 43 day cases and 48 chemotherapy treatments.
Over the past two days around 7,400 patients have been able to attend appointments, according to the health board.
Technicians have been working to restore the clinical and administrative systems at the health board since yesterday.
Scotland's Health Secretary Alex Neil instructed that a review be carried out "to make sure that these systems are robust".
He told MSPs at Holyrood that the IT problems at NHS Greater Glasgow and Clyde yesterday and today appear to have been fixed now.
Technicians have been trying to restore the clinical and administrative systems since the network servers failed yesterday.
The problems were compounded when the back-up system did not kick in, Mr Neil said.
"I can report that NHS Greater Glasgow and Clyde have now been able to resolve the problem with the server and incrementally reloading users back on to the system. It would appear that no data appears to have been lost. Around 50% of users now have access to the system, and the remainder should have access by later this evening.
"However, the system is being closely monitored to ensure it remains robust.
"Early indications are that less than 10% of outpatient appointments have been affected by this incident and there has been minimal impact on day cases and inpatient appointments," Mr Neil said.
"I understand that the failure is related to network servers that serve a number of the board's systems and sites. This meant that clinicians were unable to access some services, including some patient records and imaging.
"In addition, the recognised standby process did not kick in."
While the problems had only "minimal impact", the Health Secretary said: "I would, however, express my concern for all those patients whose treatment was affected yesterday and today."
Scottish Labour health spokesman Neil Findlay said the situation is "very worrying" because similar IT systems are used by other health boards.
"Will the Cabinet Secretary instruct an independent review of all IT systems being used, to ensure that robust contingency plans are in place across Scotland?" he asked.
Mr Neil told him: "I have already instructed a robust review right across the National Health Service in Scotland with all NHS boards to make sure that these systems are robust and the back-up systems are robust."
Mr Neil said it was still too early to be "absolutely sure as to why this has happened and why the back up system wasn't more resilient".
Staff from the two software system companies, Microsoft and Charteris, are working with both the health board and Scottish Government staff to "try to get to the root cause of the problem" he said.
The Health Secretary added: "It looks as though we've broken the back of the problem in terms of the system being back up and rebooted successfully.
"But absolutely the top priority is to get to the root of the problem so we can be absolutely sure it doesn't happen either in Glasgow or any other part of the National Health Service."
Scottish Government officials will "work closely with the board to establish the cause of this incident and to share the lesson with other NHS boards," Mr Neil said.
He also thanked the staff at the health board for their "efforts over the last two days in working to resolve this issue".
Scottish Tory health spokesman Jackson Carlaw said the problem was "not a failure of policy or management but a failure of a system".
The Conservative MSP praised the "work that has been done in order to resolve it as speedily as it has been".
Mr Neil stressed that emergency and maternity services at hospitals had been maintained throughout.
He also insisted "every effort" would be made to ensure those patients who had been affected received new appointments as quickly as possible.
Mr Neil said this could mean staff having to work longer hours "perhaps for a short period of time" to help ensure in order all patients who have had appointments or treatment delayed as seen "in a reasonable period of time".
He added he had been given a "total assurance" by the health board that "absolutely every stop will be pulled out to make sure these patients get their postponed treatment or appointment at the earliest opportunity".
Robert Calderwood, the chief executive if NHS Greater Glasgow and Clyde, had earlier issued an apology to patients who had missed treatment or appointments because of the "unprecedented IT issue".
He said: "I apologise unreservedly for the inconvenience this has caused to our patients who have had their procedures postponed and I will ensure that everything possible is done to get their treatment carried out at the earliest possible opportunity.
"I can assure everyone that our senior management team, IT specialists and clinical leads have been working around the clock and will continue to do so until this issue is resolved.
"Thanks to these efforts the vast majority of our services have been maintained and around 7,400 procedures and appointments planned over the past 36 hours have gone ahead as scheduled.
"Unfortunately however over the past 36 hours 564 patients have had their treatment or appointment postponed."
He stated: "The unprecedented IT issue relates to our network and the way staff can connect to some of our clinical and administrative systems."
Mr Calderwood said appointments or treatments had had to be cancelled because medical staff would have needed to see "detailed imaging and other patient information which was unavailable as a result of the network issue".
He added: "Arrangements have also been made to maintain the vast majority of chemotherapy sessions today although a small number of sessions have regrettably been postponed."