Backup discs may be unreadable in the Xbox drive. Different Xboxes have shipped with drives by different manufacturers, and even different drives by the same manufacturer have had varying reports as to how well that one particular drive model can read backups. Some of it may depend on the particular media being used.
Beyond this general hint about hardware, do not expect a bunch more information about this from this site, nor many of the places listed in the Other Resources page. Some webmasters may discourage talk of piracy to make sure they aren't cracked down on by legal authorities, and some webmasters may discourage discussion of piracy because they don't wish to promote behavior that they consider stealing and therefore unethical, and some webmasters may have a mixture of these feelings. Expect questions related to backups to result in harsh verbal abuse, forum postings being modified or removed, and being banned from any writing or even reading access to a website or IRC channel.
Evolution-X is the name of a group who is responsible for two entirely separate things, the RemoteX dashboard and a hacked BIOS. The RemoteX dashboard was once the only decent alternative to Microsoft's dashboard. Today other dashboards have entered the scene, and have not only many of the most important features of RemoteX (like a built-in FTP server) but other features (like a nice file manager for local use), but the Evolution-X RemoteX dashboard still has advantages such as a small file size and an ability to copy directories using scripts built into the evox.ini file.
BenJeremy says "Technically, Evolution-X is the dashboard, and Evo-X refers to the BIOS", but I doubt that many people who even know that one of the names is meant to primarily be for one of the things actually remember with certain accuracy which one is which.
Mega X-Key is a couple of things. One thing Mega X-Key is, is a memory unit with a USB connector so that it can be written to from a computer. Weighing in at 32 MB, it can store 4 times the information of an 8MB Xbox Memory Unit, such as Microsoft's official ones, Mad Catz's, and, well, any other Memory Unit designed for an Xbox that I've ever seen available. It is not needed, especially in the long term, but it may be a good idea. See Get Mega X-Key?. A Mega X-Key is probably not a bad idea, and in fact it may very well be a good approach, but it is not the only option.
The other thing that Mega X-Key is, is software that will let you write to a USB device on a computer. I do not know how compatible that software is with other devices. If you read about a certain version number of Mega X-Key, that is likely referring to the software.
In some ways, the hackers seem to have one. In other ways, the answer to this keeps changing. Hackers Versus Microsoft.
A new dashboard may add new features, such as adding support for "Xbox Live!" to a very old dashboard. It is also likely to seal up any bugs in an older dashboard. The best dashboard is likely to be the newest one that supports the features you'd like the most. One of the main characteristics of different dashboards is what bugs exist in them.
Dashboard 4920 was the newest dashboard to support the original set of dashboard hacks. When the next version of Microsoft's dashboard came out, different hacks had to be made.
I intend to check out UDE(2) more thoroughly soon.
On dashboard 4920, the font hacks and the audio hacks each have their own advantages and disadvantages. Namely the audio hacks were sure to be incompatible with having custom soundtracks stored in the standard location on an Xbox hard drive (so they could not be played by the Microsoft dashboard or any games supporting the official method of using custom soundtracks), and was arguably less convenient than the font hacks, but they allowed unhindered access to the official Microsoft dashboard and they were safer, especially from the dreaded clock loop. The audio hack needed to have A/V cables plugged into the Xbox for some unknown reason, or else the hack would not work (might it even break itself so that it would need to be re-installed?). Font hacks from this era of hacking had potential clock loop issues (especially if the Xbox did not have power going to it for some time), and may have had other issues, and were riskier in nature since they involved modifying the dashboard, but they were not incompatible with custom soundtracks and were likely safer.
On newer dashboards, there are different hacks (UDE). Some font hacks are definitely better than others. Some audio hacks are definitely better on some systems than others. More information on this will be added.
PBL loads Bootable-From-Media (BFM) BIOSes. Some BFM BIOSes disable the signature checking that the Xbox is known for. This signature checking causes the famous Error 21 screen. There are other ways to work around signature checking, such as using a mod-chip to load a BIOS or properly re-signing any XBE file before running it by using the XBEDump command. (I used to believe that XBEDump would create corrupt files when re-signing some files. I later learned that XBEDump just reports some files as being corrupt even though they aren't, so maybe maybe XBEDump is a 100% workable method, although it surely is less convenient to need to re-sign every program that goes onto the Xbox.)
PBL has had a history of not working well when changes are made to an Xbox, such as the release of the video encoder chip made by Focus and the release of Kernel 5530. The release of the Focus video encoder chip took some time for PBL and Linux to start working. With the problems introduced with the release of Microsoft's Kernel 5530, there were many months (from the time of Kernel 5530's release, through Kernel 5713's release, and well after Kernel 5838's release) that it seemed PBL would not be able to work on Kernel 5530 or newer. After some time though, PBL would be updated by somebody and the newer version(s) would then work on all the Xboxes again.
Today I believe the newest versions of PBL work on any Xbox, although I haven't looked much into that yet.
Error 21 is a screen that says the Xbox needs to be serviced, and in the upper-left corner is the number 21. I occasionally get Error 21's on my Xbox when I'm purposefully avoiding the loading of PBL and I'm being careless, or when I'm working on someone else's Xbox and being careless. Error 21 is not necessarily a problem, as one can just off an Xbox and turn on the Xbox and as long as the same procedure that caused the Error 21 isn't repeated, it won't be a problem. For instance, if running a certain file causes an Error 21, the simple solution is to simply not run that file (at least not until that file gets re-signed or signature checking is removed, such as with PBL). The time when Error 21 is really a big problem is if the bootup files have been modified in such a way that the Error 21 occurs before the user does anything, and so there is no way to prevent the Error 21.
One thing that may help is to put an official retail game in the Xbox DVD drive. It seems that depending on just how things are messed up on the hard drive, sometimes games can still run and sometimes they cannot. If games can still run, than saved game exploits and excellence can fix any situation where an Error 21 shows up just from the dashboard from the hard drive with no game in the drive. (Surely a video game rental of 007: Agent Under Fire would be cheaper than a new Xbox.)
Error 21's can be avoided by carefully following correct directions. Those with the experience and know-how can avoid Error 21's. When reading about people who say their Xbox shows an Error 21, this situation is generally caused from general stupidity (foolishness), carelessness, and/or ignorance. If one does have an Error 21 on every boot up, it can be quite difficult to fix. Or it might not be.
In the case of XSelect 0.9, I love how small it is so that it can be distributed on a memory unit. Using XSelect, it is easy to make a memory unit that can work with any one of the three saved game exploits, and it doesn't take up a lot of space to do this.
For example, ltools 1.7.5 is commonly distributed (as of this writing) in multiple packages, including one version for James Bond: Agent Under Fire, and one version for MechAssault. From my earlier exploration, it seems that with the use of XSelect 0.9, one memory unit could easily be made which can work with both 007: AUF and MechAssault, instead of needing two separate downloads and creating two separate memory units. However, to the defense of ldots, I suspect that XSelect 0.9 may be newer than ltools 1.7.5.
In short: They are useful!
These are the advantages to these things, and I think these two things haven't received as much exposure as they should have: Easter Egg Exploit should be more often used because it is safer and works with many of the other hacks too, and XSelect 0.9 is probably just a jewel not very well known about yet because it is fairly new.