In general terms, the use of proprietary cartridges was a major barrier compared to the constant cat-and-mouse game that other console manufacturers had to battle while using the CD-ROM medium.
To protect against bootleg cartridges (unauthorised reproductions), the GBA's BIOS incorporated the same boot checks as those found in the original Game Boy.
Flashcarts
As solid-state storage became more affordable, a new type of cartridge appeared on the market. Flashcarts looked like ordinary Game Paks but had the addition of a re-writable memory or a card slot. This enabled users to play game ROM files within the console. The concept is not new, actually; developers have long used similar tools internally to test their games on real hardware, with manufacturers providing the equipment to enable this.
Earlier solutions included burnable NOR Flash memory (up to 32 MB) and some battery-backed SRAM. To upload binaries to the Flashcart cartridge, the box bundled a Link-to-USB cable for use with a GBA and a PC running Windows XP. With the use of proprietary flasher software and drivers, the computer uploaded a Multiboot program to the GBA, which in turn transferred a game binary from the PC to the Flashcart (inserted in the GBA). Overall, the whole task was deemed too sluggish. So, later Flashcarts (like the 'EZ-Flash') offered larger storage and could be programmed without requiring the GBA as an intermediate . Finally, later revisions migrated to removable storage (SD, MiniSD, MicroSD, and whatnot).
Commercial availability of these cards proved to be a grey legal area: Nintendo condemned their use for enabling piracy, whereas some users argued that it was the only method for running Homebrew (programs developed outside game studios and consequently without the approval of Nintendo). The company's argument was supported by the fact that flashers like the EZ-Writer assisted users in patching commercial ROMs, so they could run on EZ-Flash carts without issues. Following Nintendo's legal action, these cartridges were banned in some countries (such as the UK). Nonetheless, they persisted worldwide.