/mnt/sda1 is where an SD card is usually mounted, although it could also appear as /mnt/sda2 or higher, depending on circumstances.
Because there is sometimes some variability in the mount point, the system looks at a recently mounted SD card, and if it contains a folder named arduino at the root of the SD card, it will create a symbolic link to the SD card at /mnt/sd.
Using /mnt/sda1 will usually work, but sometimes you will need to use /mnt/sda2 or something else.
Using /mnt/sd should always work, as long as you have the folder named arduino at the SD card root. If you don't have it, create it, but the /mnt/sd link probably won't be created until you re-insert the card after creating the folder.