USB2 is easily fast enough for HD replay (including encrypted HD content). The type of disc (IDE/SATA) inside a usb enclosure makes no difference, though these days you will find it hard to find a ide drive. You are probably getting confused with uncompressed HD formats, mpeg compression (h264/AVC for HD) reduces the transmission bitrate well within the capability of usb.
The HD FOX T2 freeview box can record HD directly to a USB drive without difficulty.
Only FAT (Max file size 4GB) and EXT3 is supported (not EXT2, early references to EXT2 were incorrect)
HD material recorded in Freesat mode is normally copy once. Onve copied to usb you can replay the copy from the usb drive.
If you enter non-freesat and reboot, you can manually initiate HD recordings which will be without copy restrictions or encryption.