If casting in small areas a witch will often cast to the walls of the room thus embracing the entire room in protected space. It can be very difficult in these circumstances to remove every piece of electronic equipment from the room. You need to think how practical it is to empty everything out of the room. If I had to disconnect and remove from the room every tarantula heat mat every time I wanted to complete circle work I’d probably get so fed up I’d never cast again!
My own view is that you have to take a sensible approach. Mobile phones should always be removed as there is always the danger of them ringing and breaking concentration. Mobiles also operate on a very high frequency (similar to televisions) and can be particularly disruptive to other energies. I would always switch off or remove a computer as well as I personally dislike the idea of having a communication device or anything that could connect with energy outside the protected space in my circle. I’d extend this to radios as well but would find a cd player or an mp3 player/speaker acceptable.
I’ve not had a problem with this personally but some witches have reported the energy of their circle breaking nearby electronics. This is another reason why you may have heard magic and electrics should be separated. The most common issue seems to be with watches – many witches refuse to wear a watch when casting as it is very common for watches to break during circle or spell work.
(I did manage to crack a diamond down the middle once and have always suspected this was due to the energies involved in casting so it might be worth removing any precious jewellery along with the electronic equipment.)