RouterBOARD uses NAND as hard drive, external drive is more recommended due to space limitation 64/128/512 MB, when HTTP caching is necessary on your network.
External drive on RouterBOARD
The list of the RouterBOARDs, where is the option to add external drive;
RouterBOARD 5xx (532/532A - Compact Flash card slot);
RouterBOARD 4xx (433AH - MicroSD card slot; 433UAH - MicroSD card slot; 450G - MicroSD card slot
RouterBOARD 6xx (600/600a - Compact Flash card slot);
RouterBOARD 1000 (1000 - Compact Flash card slot);