1) You need to make sure there are no partitions at all on the SD card, so the only space on there is shown as unallocated - I use EaseUS Partition Master to do this but it's important to only use it to do this, don't be tempted to use it to format the card as further down the line it'll give you problems.
2) With the now 'partition free' SD card put it in the receiver and boot it up. Once booted make sure it has not been mounted (you won't be able to use the format tool for something that's already mounted), so Blue > Mounts > Device Manager and use Unmount (yellow button) if required.
3) Once unmounted head to my SD Format Plugin (in Plugin Browser) and let it run. It'll take a while, SD cards just do and usual thing the bigger the storage space the longer it'll take, but in the end it'll get there and then the receiver will reboot.
4) Once it boots back up head back to Device Manager again, you should see the card there listed as ext4 R/W and probably pre-mounted but you will probably want to change this to /media/hdd (pre-defined paths for recording and timeshift) so press the red button for 'use as HDD' - This should update the mount to /media/hdd.
5) You are almost there but an important step here now - Give the receiver a full reboot via the menu now, not just GUI restart, it wan't to be a full restart but via menu rather than just pulling power.
Hopefully if all went well when the receiver boots back up now you will have the SD card mounted as /media/hdd, the reboot will have created two folders on there (movie and timeshift) and you should be ready to go for both recording and timeshift, you can also use this for EPG data as well if you wish.