Features

Card security (non-DRM)

Read-only cards

The host device can command the SD card to become read-only (to reject subsequent commands to write information to it). There are several different ways to achieve this, one of which is irreversible.
Using a read-only device in Microsoft Windows may produce errors, as even when merely reading a file, Windows tries to write a timestamp to the device.

Write protection tab

When looking at the card from the top, the right side (the side with the beveled corner) must be notched.
On the left side, there may be a write-protection notch. If the notch is omitted, the card can be read and written. If the card is notched, it is read-only, except that the notch may be partly covered by a sliding tab. In this case, the user can slide the tab upward (toward the contacts) to declare the card read/write, or downward to declare it read-only.
The presence of a notch, and the presence and position of a tab, have no effect on the SD card's operation. A host device that supports write protection should refuse to write to an SD card that is designated read-only in this way. Some host devices do not support write protection, which is an optional feature of the SD specification. Host devices that do obey a read-only indication may give the user a way to override it. The miniSD and microSD formats do not support write protection with the notch-and-tab method.
Typical cards sold as a medium for protected content are permanently marked read-only by having a notch and no sliding tab.

Card password

A host device can lock an SD card using a password of up to 16 bytes, typically supplied by the user. A locked card interacts normally with the host device except that it rejects commands to read and write data. A locked card can be unlocked only by providing the same password. The host device can, after supplying the old password, specify a new password or disable locking. Without the password (typically, in the case that the user forgets the password), the host device can command the card to erase all the data on the card for future re-use (except card data under DRM), but there is no way to gain access to the existing data.

DRM features

All SD cards incorporate a digital rights management (DRM) scheme. Roughly 10% of the storage capacity of an SD card is not available to the user, but is used by the on-card processor to verify the identity of an application program that it will then allow to read protected content. The card prohibits other accesses, such as users trying to make copies of protected files.
The DRM scheme embedded in the SD cards is the Content Protection for Recordable Media (CPRM or CPPM) specification of the 4C Entity, which features the Cryptomeriera ciph (also termed C2). The specification is kept secret and is accessible only to licensees. The scheme has not been broken or hacked, but this feature of SD cards is rarely used to protect content. DVD-Audio uses the same DRM scheme.
Windows Media files can be DRM-encoded so as to make use of the SD card's DRM abilities. 

Windows Phone 7 devices use SD cards designed to be accessed only by the phone manufacturer or mobile provider. An SD card inserted into the phone underneath the battery compartment becomes locked "to the phone with an automatically generated key" so that "the SD card cannot be read by another phone, device, or PC". Symbian devices, however, are some of the very few which can perform the necessary low-level format operations on locked SD cards. It is therefore possible to use a device such as the Nokia N8 to reformat the card for subsequent use in other devices.
The Super Digital cards manufactured by Super*Talent are the same in appearance and function as Secure Digital cards, but they lack the CPRM feature of Secure Digital cards.