She said: "There was no warning, no explanation and it was really frightening for all of us waiting to go in. I still get tearful thinking about it now."
Reader - who has since been returned to Belmarsh Prison - is one of seven gang members waiting to be sentenced at Woolwich Crown Court next week.
His legal team do not know whether he will be well enough to attend court and may ask to have his hearing postponed.
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Reader and three others - Danny Jones, Terry Perkins and John Collins - pleaded guilty to conspiracy to burgle the Hatton Garden Safe Deposit vault over the Easter weekend last year.
The gang drilled through a concrete wall and stole £14m worth of jewellery, gold and cash, most of which is still missing.
For their guilty pleas, the ringleaders should get a third off the maximum 10-year jail sentence and they will have to serve only half of the time.
With another 10 months deducted for time served since their arrest, they could be freed in little more than two-and-a-half years.
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But they are likely to spend many more years in jail because of penalties under the Proceeds of Crime Act.
At a later hearing they will be asked to pay back the missing £10m and if they don't, they could be jailed for up to another 14 years, with no remission, on top of the burglary sentence.
Former chief prosecutor Nazir Afzal said: "There is a lot of stolen property missing and the judge will ask them to give it back. If they don't, they will get hit really hard. They could spend 17 years in prison."
Two other men - Carl Wood and William Lincoln - who pleaded not guilty but were convicted of two charges could get even longer sentences.
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Plumber Hugh Doyle, 48, who was convicted of laundering the stolen loot after letting the gang use his workshop forecourt to transfer bags of jewellery between vehicles, said he was hoping to be given a time-served sentence and walk free from court.