Hi all, this is a UK perspective....
in 10yrs of digital multimedia evidence recovery, I have only had to pull to the legislation card twice! As a police officer lawfully on premises I have a power under the Police and Criminal Evidence Act that enables me to seize property that is, or contains, evidence. If the DVR is unable to export the evidential video then we do quite regularly take the whole unit in order for the recovery to take place in the lab.
When then DVR owner is not connected to the incident, then it is our responsibility to ensure that the premises is not left without protection so we have a number of 'swap out' units. We put these in place whilst we have the DVR in our possession.
If we are likely to have it for some time, or in major cases where it has recorded an incident of very high value, then we liase with the owner or installer to have a new one fitted and the investigation foots the bill!
The only time we seize and don't replace is when the DVR owner is suspected of tampering, or is suspected to tamper, if they are left with it. Also when the DVR owner is a suspect/offender.
Obviously, we would not go down the 'swap out' route on low level crime. In those circumstances we may just take a stream out of a small incident or state that it is not in the publics interest to remove an entire unit for a small value theft.
As people beforehand have stated, it is a poor installation that does not take evidential export into consideration. A DVR must be able to export data that is the same as was recorded, and it must be able to export any amount - 10mins, 10 hrs, or the whole lot!
Also, when people refer to off site storage - this must not be a stream transcode of the original. Any change in the digital make up jeapordises it's evidential worth.
Hope that helps, and thanks IPVM for allowing me to post.