The general consensus is that the ideal HDD to go for runs at 5400rpm, cache shouldn’t be a concern. The majority of successful upgrades have been done with primarily Maxtor drives but also Samsung. More recently 7200rpm drives have been shipping in new boxes from Sky, and so some 7200rpm drives have been used for upgrades also.
Please note that not every make or model of HDD works in a Sky+ box, probably due to timing differences and feature sets provided by the different drive manufacturers. Some drives will work well in some makes/models of Sky+ box, whilst giving unreliable recording in others. It is therefore highly recommended to obtain a drive that has been tried and tested for your particular make and model of box. That’s not to say that an untested drive will definitely not work, just that you will be taking a chance by ordering one and just expecting it to work flawlessly.
