Logically, your best approach would be to appeal to the attorney\'s sensibilities to keep it confide
I don\'t think you have any option but to tell the attorney. They respect privacy issues and would best understand (though they may be legally tied) your concerns about your employer coming to know this, esp if the arrest has no bearings on your job performance. After all, if the 485 filing had not occurred your company wouldn\'t come to know of this.
I WOULD DEFINITELY NOT GO TO HR ! HR\'s job is to act in the the company\'s interest, usually has little clout and will definitely not keep it confidential. Your best bet is the external attorney - I\'m sure they have encountered this before. Also, are you sharing the attorney fees with your company. Then all the more reason they\'ll feel obliged to look after your interests.
Another option is after you call have your own personal attorney make a follow-up courtesy call (mind you, no threats) to this immigration attorney to represent your concerns. He/she will be best able to represent your interest in \'legalese\', maybe even establish a rapport.
The reason I\'m suggestin this is I once had to get my personal attorney to call my company attorney on a completely unrelated matter.He was able to express my concerns and make a request on my behalf in a way I would not have been able to. It was worth it.
Hope this helps. Do let us know how things fared. Good luck!