The Game Boy Advance, like its predecessor line, does not have any intrinsic copy protection - only a Nintendo logo and header checksum verification; in fact, what incidentally turned out to be a more effective obstruction against unlicensed/copied games was the high performance of the game card bus, which could not be matched by affordable high capacity flash storage of the time. GBA ROMs did however also have "smarts" built into them (for sequential access acceleration, etc), making them not directly replaceable with general purpose [[[E]E]P]ROM.
The various and fundamentally incompatible save storage technologies used across different commercial titles, coupled with novelty hardware that could be connected via the bus, also effectively obstructed the development of a truly universal Game Pak emulator - although many would agree a satisfactory job is done by current market offerings.
The first third-party rewritable GBA cards was presumably the Visoly FlashAdvance series, launched in 2002. They could only store one program, although multi-game building software was created to address this limitation (with mixed results).
Some models supported the GB Bridge, a mechanical adapter and level shifter allowing them to be reused as 8-bit Game Boy flashcards.
These were flashcards in the strict sense, requiring the use of an external programmer, often requiring proprietary Windows software and a contemporary operating system with few to no restrictions on unsigned drivers and direct I/O; said software also applied compatibility hacks such as save type patching. Fortunately, some modern card readers, multiboot-based GBA programming software, and even homebrew for the DS/Lite such as "FAS1 Linker" has some support for these.
Of today's brands, the EZ-Flash 1 and 2 represent this category.
Built-in storage age
As one of the main advantages of a flashcard, by today's expectations, is to carry on a single piece of media all the legal homemade backups of your legally owned games, technology had to march on.
A second generation of flashcards appeared on the market, having a relatively large NAND to hold the library and a high performance NOR on which the desired program would be copied to and run from.
Most of these still need an external programmer (as is the case with the EZ-Flash 3 and the FlashAdvance Pro), an USB interface on the card itself (seen on the EFA Linker), or an USB to GBA serial cable (typical of the Flash2Advance). Game patching continued to be the norm until the 2015 scene revival.
Switch to removable storage
Eventually the main storage started being omitted and replaced with some removable storage device, typically a CompactFlash, SD, or miniSD; proprietary software stopped being required for transferring games, although patching was still a necessity for most titles.
The EZ-Flash 4 (miniSD), EWin, and other reputable brands, some of which went on to the DS scene, represented this period (which peaked sharply around 2006 and ended by 2010).
Many of these eventually doubled, in combination with a PassMe or equivalent, as Slot-2 DS flashcards. However, as bus performance expected by DS software was lower, those designed especially for this use (such as the SuperCard, which unfortunately was the only widely available GBA flashcard in the first half of the 2010s, and the G6) often turned out to be unfit for GBA software, and the EZ-Flash Lite Compact did not even nominally support it. To its credit, the SuperCard was the longest-updated Slot-2 flashcard, with a compatibility list sporting some 2010 titles.
The ubiquitous NOR storage, which as mentioned was a conspicuous part of a flashcard's hefty price, tended to be supplemented or (rarely) even replaced by PSRAM which also greatly improved programming (game loading) times, but required the process to be repeated after a power cycle.
Aside - GBA flashcards for DS
Some devices, not advertised for use in an actual GBA, had no "long term" storage at all. They were designed to complement the more modern and superior Slot-1 DS flashcards which cannot (satisfactorily) be used for GBA software.
These products, such as the EZ-Flash 3in1, the M3 GBA Extension Pack, and the EWin GBA Extension Pack, also tend to offer rumble and/or additional memory for the DS.
But if the user decides to use their NOR storage, they can also approximate a first-generation GBA flashcard, only requiring a DS/Lite to change games. While therefore few would have wanted one for use outside of a DS, new old stock DS-Lite sized 3in1s happened to be the only GBA flashcard of reasonable quality (after battery replacement) available around 2014.
Additionally, since the programmed ROM would be exposed directly on the GBA bus without any loader, they can be used for GameCube and DS titles which expect a certain game in the GBA slot (but not if they need to read a non-SRAM save).
The return of EZ-Flash
In 2015, the EZ-Flash team (which previously dabbled with good quality but ultimately abandoned Slot-1 products, the EZ-Flash 5 and 6, and later even the Redux, a Gateway knockoff) acclaimedly reopened the GBA scene in 2015 with a remake of their flagship card, the EZ-Flash 4, now with a black case, microSD drive, and SDHC support (which was also backported to the miniSD version).
In the meanwhile, independent flashcard designer Krikzz had come to fame for his EverDrive products for other consoles; in 2016, he launched the GBA EverDrive X5, which quickly became appreciated as an upper-end product compared to the mass-market EZ-Flash: while it cost over twice its competition while also being larger, it was the first GBA flashcard to have an RTC in a decade, and the first designed to run clean ROMs directly.
Not to be outdone, the EZ-Flash 4 received a major update in mid 2017, offering on-demand patching and therefore clean ROM support, at the expense of increased loading time. The team followed up with the EZ-Flash Reform, a miniature version of the EZ-4 which fits almost perfectly in a DS Lite while also having a non-soldered battery, and bundling a conventionally sized shell for use on other consoles.
2018 was the year of the EZ-Flash Omega, a new generation card which kept the Reform's double form factor (but not its interchangeable battery) offering a clock, fast patching and loading to PSRAM (approx. 5 seconds) for known games, and the removal of SRAM in favor of actual save chip emulation, doing away with the near requirement for a good battery. However, various combinations of user ignorance, lack of SD card access LED (a mildly-popular aftermarket mod), some titles' use of autosave, and SDs of unknown performance (exacerbated by this being a good application for those leftover small capacity ones) lead to uncertainty about whether it is safe at any specific time to turn off the power, with a non-negligible amount of save and filesystem corruption complaints.
In 2020 another round was exchanged by the competitors with the mid-year launch of the GBA EverDrive Mini, which shrunk its predecessor to standard GBA size with no other changes or tradeoffs; and for the Christmas season of EZ-Flash Omega Definitive, a premium version of its predecessor which returned to a more conventional FRAM+backup save system, reintroduced the advantages of the DS expansion cards (including rumble), a non-soldered battery, power/access LED, as well as being the first flashcard to emulate 64 MB ROMs and their mapper.
- There were, unsurprisingly, developer-exclusive flashcards licensed by Nintendo
- Outside of the DS expansion cards; but technically, the patch was performed by the DS-mode loader.